[libc][NFC] Add a platform independent thread support library.

The idea is that, other parts of the libc which require thread/lock
support will be able to use this platform independent setup.

With this change, only the linux implementation of a mutex type has been
moved to the new library. Because of this, there is some duplication
between the new library and src/threads/linux. A follow up change will
move all of src/threads/linux to the new library. The duplication should
be eliminated with that move.

Reviewed By: lntue

Differential Revision: https://reviews.llvm.org/D120795