unix: workaround broken pthread_sigmask on Android

Introduce `uv__sigmask` which is defined to be `sigprocmask` on Android
and `pthread_sigmask` on all other platforms. This is to workaround 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

`uv__sigmask` should be used in place of `pthread_sigmask`.