[libc] Rename pthread_impl.h to threads_impl.h

Baby step towards separating pthreads out of libc

Change-Id: Ibb1ba6656c796218909f4a3451db9bb66b99fb80

[musl] Remove duplicate declaration of __clock_gettime

The other is in pthread_impl.h and already has the correct visibility.

Change-Id: Ieafb7dd9ef7667cdce1ab56aa9c76e74cde2a977

[musl] Move __clock_gettime declaration into a header and dedup a macro

Change-Id: I9fd466b2b58c2bab7f528976373a7b1cd9d8e773

[ulib][musl] Clean up clock* stubs, drop timer_* functions

We have some of the POSIX.1 clock_* interfaces, but don't fully
support all of them. Make the ones we don't support be simple
ENOSYS stubs instead of calling into nonexistent Linux hooey.

We don't support the POSIX.1 timer_* interfaces at all, so don't
build that code at all. If we ever support these interfaces,
they will need a wholly new implementation anyway.

Change-Id: I446608c9011938b50ce7f23af08de5f61b3cfb96

[musl] clang format musl

Change-Id: I3bc6ab7655410514691cd07f08454f3afd41d9b3

[magenta] Initial commit