[libc][NFC] Move the mutex implementation into a utility class.

[libc][NFC] Move the mutex implementation into a utility class.

This allows others parts of the libc to use the mutex types without
actually pulling in public function implementations.

Along the way, few cleanups have been done, like using a uniform type to
refer the linux futex word.

Reviewed By: michaelrj

Differential Revision: https://reviews.llvm.org/D108749