unix: workaround broken pthread_sigmask on Android

On android, if a call to `pthread_sigmask` fails with EINVAL, fallback
to `sigprocmask`. This workarounds the broken behaviour of
`pthread_sigmask` on Android versions < 4.1. See:

* https://android.googlesource.com/platform/bionic/+/9bf330b5
* https://code.google.com/p/android/issues/detail?id=15337

This is implemented by replacing all calls of `pthread_sigmask` to a new
function `uv__pthread_sigmask` which implements this fallback mechanism.