x/sys/unix: TestPrctlRetInt failures with "unexpected return from prctl" on android-amd64-emu builder

[bcmills]

--- FAIL: TestPrctlRetInt (0.00s)
    syscall_linux_test.go:827: unexpected return from prctl; got 0, expected 1
FAIL
exitcode=1
FAIL	golang.org/x/sys/unix	62.852s

greplogs --dashboard -md -l -e 'FAIL: TestPrctlRetInt'

2022-01-13T23:35:37-5a964db-e550c30/android-amd64-emu
2021-09-29T13:53:55-39ccf1d-e180e2c/android-amd64-emu

This is the test added in CL 209969 for #35978 (CC @tklauser @darkLord19).

From man prctl, it appears that the return value from PR_GET_NO_NEW_PRIVS depends on the state of the calling thread, so probably this test needs to be executed on a thread-locked goroutine.

[gopherbot]

Change https://golang.org/cl/378555 mentions this issue: unix: lock the thread for TestPrctlRetInt