MainLoop: work around an android libc bug

Versions of android before kitkat implemented pselect non-atomically,
which caused flakyness, as we were relying on it atomically setting the
signal mask to implement waiting for signals.

This patch implements a direct call to the the pselect kernel syscall,
which does not suffer from this problem. The code itself is not very
pretty, but fortunately the uglyness is contained in the
android version of the MainLoop::RunImpl::Poll function.

llvm-svn: 316915