runtime: TestBadOpen failure on netbsd-arm-bsiegert

[bcmills]

From the netbsd-arm-bsiegert builder (https://build.golang.org/log/8ecaff969fb69a693ea224a6891ac4e12cfd9d17):

--- FAIL: TestBadOpen (0.00s)
    crash_unix_test.go:48: read()=9, want -9
    crash_unix_test.go:52: write()=9, want -9
FAIL
FAIL	runtime	135.292s

That builder has been AWOL for a while, so it's not yet clear whether the failure is reproducible, but the test was introduced recently (in CL 171823).

CC @bsiegert @ianlancetaylor @mknyszek @dvyukov

[gopherbot]

Change https://golang.org/cl/202441 mentions this issue: runtime: don't negate errno value for netbsd-arm read/write