Resurrect pselect MainLoop implementation

Summary:
It turns out that even though ppoll is available on all the android
devices we support, it does not seem to be working properly on all of
them -- MainLoop just does a busy loop with ppoll returning EINTR and
not making any progress.

This brings back the pselect implementation and makes it available on
android. I could not do any cmake checks for this as the ppoll symbol is
actually avaiable -- it just does not work.

Reviewers: beanz, eugene

Subscribers: srhines, lldb-commits

Differential Revision: https://reviews.llvm.org/D32600

llvm-svn: 301636