[libc] Rename pthread_impl.h to threads_impl.h

Baby step towards separating pthreads out of libc

Change-Id: Ibb1ba6656c796218909f4a3451db9bb66b99fb80

[musl] Remove unneeded _ALL_SOURCE definition

Change-Id: I14e0cb6a78b70fd0371bd4e680ca14ddb92509b8

[musl][threading] Prefer __thrd_current in non-pthread code

Change-Id: I4f4e6da25df65e9c6660e7f7dc3a3aeee5a309a7

[musl] Add implementation of __cxa_thread_atexit_impl

C++ ABI expects this function to be provided by the C library, libcxxabi
it has its own implementation that is used in case when C library
doesn't provide one, but that implementation has number of limitations
and pulls in additional dependencies which are undesirable
(__dso_handle).

This change adds implementation of __cxa_thread_atexit_impl into our C
library. This version is resembles the implementation __cxa_atexit, and
is significantly simpler than implementations of
__cxa_thread_atexit_impl in other C libraries because our C library
doesn't support dlclose.

MG-655

Change-Id: I1f79853f9cff3b8ecf9c35419f74bf3463f3df58