[libc] Rename pthread_impl.h to threads_impl.h Baby step towards separating pthreads out of libc Change-Id: Ibb1ba6656c796218909f4a3451db9bb66b99fb80
[musl] Make thrd_{create,exit,join} call non-public pthread functions Change-Id: Id64a71f3210d8891b598b7c02d22ecfc65784c77
[musl] Move C11 thread create and exit to the real pthread implementations Change-Id: I0b8a81d7a15ea5655132e32d09fa8e9194dcb2fb
[magenta] Initial commit