x/sys/unix: TestPselect flaky on some builders

[tklauser]

TestPselect occasionally fails with:

syscall_linux_test.go:301: Pselect: interrupted system call

linux-386: https://build.golang.org/log/d2ac4e02833b0129a9b11f5d3ea0f122d1813baf
linux-amd64-nocgo: https://build.golang.org/log/4f61e651ce23e678c12f88df80524ed7cd28c6e7
android-amd64-emu: https://build.golang.org/log/14519f68dd1a83e8868f110d3a31c6ceddf9f141

The easiest fix is probably to just skip TestPselect if Pselect returns EINTR.

/cc @ianlancetaylor @bradfitz

[gopherbot]

Change https://golang.org/cl/206862 mentions this issue: unix: skip TestPselect on EINTR

[gopherbot]

Change https://golang.org/cl/207284 mentions this issue: unix: don't fail TestPselect on EINTR in zero Pselect case

[gopherbot]

Change https://golang.org/cl/207292 mentions this issue: unix: fix EINTR check in TestPselect