runtime: failed to create new OS thread when building toolchain2 on openbsd-mips64-jsing since CL 447175

[bcmills]

Since https://go.dev/cl/447175, the openbsd-mips64-jsing builder has been failing every run for the main Go repo.

As far as I can tell the failure mode is always failed to create new OS thread while building toolchain2,
but the errno value varies from run to run (and seems to be bogus).

https://build.golang.org/log/f6d3242e8e0d9549fbe403a7d40dad494e90688b:

Building Go toolchain2 using go_bootstrap and Go toolchain1.
runtime: failed to create new OS thread (have 1 already; errno=-394359)
fatal error: runtime.newosproc

runtime stack:
runtime.throw({0x5547ea, 0x11})
	/home/gopher/build/go/src/runtime/panic.go:1047 +0x58 fp=0xfffffc2568 sp=0xfffffc2540 pc=0x55c60
runtime.newosproc(0xc00004c000)
	/home/gopher/build/go/src/runtime/os_openbsd_syscall.go:49 +0x1f4 fp=0xfffffc25d8 sp=0xfffffc2568 pc=0x525e4
runtime.newm1(0xc00004c000)
	/home/gopher/build/go/src/runtime/proc.go:2244 +0x110 fp=0xfffffc2610 sp=0xfffffc25d8 pc=0x5ed98
runtime.newm(0x5909c8, 0x0, 0xffffffffffffffff)
	/home/gopher/build/go/src/runtime/proc.go:2219 +0x1b8 fp=0xfffffc2640 sp=0xfffffc2610 pc=0x5ec18
runtime.main.func1()
	/home/gopher/build/go/src/runtime/proc.go:171 +0x54 fp=0xfffffc2660 sp=0xfffffc2640 pc=0x5907c
runtime.systemstack()
	/home/gopher/build/go/src/runtime/asm_mips64x.s:189 +0x80 fp=0xfffffc2668 sp=0xfffffc2660 pc=0x933b8

goroutine 1 [running]:
runtime.systemstack_switch()
	/home/gopher/build/go/src/runtime/asm_mips64x.s:150 +0x8 fp=0xc000048780 sp=0xc000048778 pc=0x93318
runtime.main()
	/home/gopher/build/go/src/runtime/proc.go:170 +0x98 fp=0xc0000487d8 sp=0xc000048780 pc=0x59128
runtime.goexit()
	/home/gopher/build/go/src/runtime/asm_mips64x.s:624 +0x4 fp=0xc0000487d8 sp=0xc0000487d8 pc=0x9575c
go tool dist: FAILED: /home/gopher/build/go/pkg/tool/openbsd_mips64/go_bootstrap install cmd/asm cmd/cgo cmd/compile cmd/link: exit status 2

(attn @4a6f656c @golang/openbsd; CC @ianlancetaylor @mknyszek)

[ianlancetaylor]

I assume that the fix is https://go.dev/cl/451257.

[bcmills]

Fixed by CL 451257.