[libc] Rename pthread_impl.h to threads_impl.h

Baby step towards separating pthreads out of libc

Change-Id: Ibb1ba6656c796218909f4a3451db9bb66b99fb80

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

Change-Id: I4f4e6da25df65e9c6660e7f7dc3a3aeee5a309a7

[ulib][musl] Provide __get_unsafe_stack_start and __get_unsafe_stack_ptr

Clang supports builtins called __builtin___get_unsafe_stack_start
and __builtin___get_unsafe_stack_ptr that just call these library
functions (even though LLVM knows how to get the unsafe SP itself).

Change-Id: I375c6e6a79bc552e6decd9164c57ff706ab48314