runtime: continuing TestSegv/SegvInCgo failures with "unknown pc"

[bcmills]

#!watchflakes
post <- pkg == "runtime" && test == "TestSegv" && `SegvInCgo` && `unknown pc`

--- FAIL: TestSegv (0.00s)
    --- FAIL: TestSegv/SegvInCgo (0.15s)
        testenv.go:361: [/var/gobuilder/buildlet/tmp/go-build403866584/testprogcgo.exe SegvInCgo] exit status: exit status 2
        crash_cgo_test.go:596: SIGSEGV: segmentation violation
            PC=0x6e267e74 m=2 sigcode=0
            
            goroutine 0 [idle]:
            runtime: unknown pc 0x6e267e74
            stack: frame={sp:0x5d9cddd8, fp:0x0} stack=[0x5d5ce388,0x5d9cdf88)
            0x5d9cdd58:  0x00122318  0x5d9cddf4  0x5d9cdd84  0x5d9cdd70 
            0x5d9cdd68:  0x6e2eb950  0x6e2e8380  0x6dec0000  0x00000000 
            0x5d9cdd78:  0x5d9cdda4  0x5d9cdd88  0x6e2eba88  0x6e2e90a8 
            0x5d9cdd88:  0x000f4240 <net.(*resolverConfig).tryUpdate+0x00000264>  0x5d9cddec  0x01406f40  0x6df6a000 
            0x5d9cdd98:  0x5d9cdde4  0x5d9cdda8  0x001222a0  0x6e2eba20 
            0x5d9cdda8:  0x5d9cdddc  0x5d9cddb8  0x6e1b3088  0x6e1f7b30 
            0x5d9cddb8:  0x00000001  0x5d9cddf4  0x5d9cde10  0x6df6a000 
            0x5d9cddc8:  0x0021da48  0x0042e240  0x000000f0  0x004025a0 
            0x5d9cddd8: <0x5d9cde3c  0x5d9cdde8  0x00122470  0x6e2e9838 
            0x5d9cdde8:  0x000000f0  0x6dec0000  0x00400000  0x22220002 
            0x5d9cddf8:  0x00000000  0x00000000  0xffffffff  0xffffffff 
            0x5d9cde08:  0xffffffff  0xffffffff  0x00000000  0x00000000 
            0x5d9cde18:  0x00000000  0x00000000  0x00000000  0x5d9e621e 
            0x5d9cde28:  0x00000040  0x00000000  0x0021dbd8  0x5d9cde40 
            0x5d9cde38:  0x0007b888 <runtime.asmcgocall+0x000000ac>  0x001223e8  0x00245974  0x00000000 
            0x5d9cde48:  0x00000001  0x00423901  0x5d9cde5c  0x00000000 
            runtime: unknown pc 0x6e267e74
            stack: frame={sp:0x5d9cddd8, fp:0x0} stack=[0x5d5ce388,0x5d9cdf88)
            0x5d9cdd58:  0x00122318  0x5d9cddf4  0x5d9cdd84  0x5d9cdd70 
            0x5d9cdd68:  0x6e2eb950  0x6e2e8380  0x6dec0000  0x00000000 
            0x5d9cdd78:  0x5d9cdda4  0x5d9cdd88  0x6e2eba88  0x6e2e90a8 
            0x5d9cdd88:  0x000f4240 <net.(*resolverConfig).tryUpdate+0x00000264>  0x5d9cddec  0x01406f40  0x6df6a000 
            0x5d9cdd98:  0x5d9cdde4  0x5d9cdda8  0x001222a0  0x6e2eba20 
            0x5d9cdda8:  0x5d9cdddc  0x5d9cddb8  0x6e1b3088  0x6e1f7b30 
            0x5d9cddb8:  0x00000001  0x5d9cddf4  0x5d9cde10  0x6df6a000 
            0x5d9cddc8:  0x0021da48  0x0042e240  0x000000f0  0x004025a0 
            0x5d9cddd8: <0x5d9cde3c  0x5d9cdde8  0x00122470  0x6e2e9838 
            0x5d9cdde8:  0x000000f0  0x6dec0000  0x00400000  0x22220002 
            0x5d9cddf8:  0x00000000  0x00000000  0xffffffff  0xffffffff 
            0x5d9cde08:  0xffffffff  0xffffffff  0x00000000  0x00000000 
            0x5d9cde18:  0x00000000  0x00000000  0x00000000  0x5d9e621e 
            0x5d9cde28:  0x00000040  0x00000000  0x0021dbd8  0x5d9cde40 
            0x5d9cde38:  0x0007b888 <runtime.asmcgocall+0x000000ac>  0x001223e8  0x00245974  0x00000000 
            0x5d9cde48:  0x00000001  0x00423901  0x5d9cde5c  0x00000000 
            
            goroutine 1 [sleep]:
            time.Sleep(0x3b9aca00)
            	/var/gobuilder/buildlet/go/src/runtime/time.go:194 +0x170
            main.SegvInCgo()
            	/var/gobuilder/buildlet/go/src/runtime/testdata/testprogcgo/segv.go:56 +0xcc
            main.main()
            	/var/gobuilder/buildlet/go/src/runtime/testdata/testprogcgo/main.go:34 +0x158
            
            goroutine 6 [runnable]:
            main._Cfunc_nop()
            	_cgo_gotypes.go:364 +0x38
            main.SegvInCgo.func1()
            	/var/gobuilder/buildlet/go/src/runtime/testdata/testprogcgo/segv.go:47 +0x20
            created by main.SegvInCgo
            	/var/gobuilder/buildlet/go/src/runtime/testdata/testprogcgo/segv.go:44 +0x74
            
            trap    0x0
            error   0x0
            oldmask 0x0
            r0      0x0
            r1      0x0
            r2      0x0
            r3      0x0
            r4      0x0
            r5      0x5d9cde10
            r6      0x6df6a000
            r7      0x21da48
            r8      0x42e240
            r9      0xf0
            r10     0x4025a0
            fp      0x5d9cdde4
            ip      0x6e300674
            sp      0x5d9cddd8
            lr      0x6e2e983c
            pc      0x6e267e74
            cpsr    0x400d0010
            fault   0x0
            
        crash_cgo_test.go:622: unexpectedly saw "runtime: " in output
FAIL
FAIL	runtime	155.487s

greplogs --dashboard -md -l -e '\Anetbsd-.*(?:\n.*)*FAIL: TestSegv/SegvInCgo .*(?:\n .*)*unknown pc' --since=2022-01-07

2022-02-01T16:10:04-93fe469/netbsd-arm-bsiegert

It is not obvious to me whether this has the same underlying cause as #50605.
(See previously #49182; CC @prattmic @cherrymui.)

[bcmills]

This failure mode has now also occurred on linux-amd64-clang, and linux/amd64 is a first class port.

Given how rare the failure seems to be and how little time is left in the Go 1.18 cycle, marking as release-blocker for Go 1.19 (instead of 1.18).

greplogs --dashboard -md -l -e 'FAIL: TestSegv/SegvInCgo .*(?:\n .*)*runtime: unknown pc' --since=2022-01-07

2022-02-04T22:34:05-f9763a6/linux-amd64-clang
2022-02-01T16:10:04-93fe469/netbsd-arm-bsiegert
2022-01-13T23:35:37-e550c30/linux-mips64le-mengzhuo

[bcmills]

greplogs --dashboard -md -l -e 'FAIL: TestSegv/SegvInCgo .*(?:\n .*)*runtime: unknown pc' --since=2022-02-07

2022-02-10T15:23:05-452f24a/linux-riscv64-unmatched

[bcmills]

Four flakes in a month during the testing-lull that is the code freeze makes me think this test is too noisy to leave enabled in the runtime package.

If we aren't going to be able to fix it ahead of the 1.18 release — especially given that one of the failures was observed on a first-class port — I think it at least needs a skip.