[libc] Rename pthread_impl.h to threads_impl.h

Baby step towards separating pthreads out of libc

Change-Id: Ibb1ba6656c796218909f4a3451db9bb66b99fb80

[musl][condvar] Clean up signature of internal condvar signal function

First, make this return void, since it never fails. Second, share the
declaration in a header rather than declaring extern it in several
places.

Change-Id: I1be37b9c7ea5cb2374494b6a248b87a6815c6b57

[magenta] Initial commit