[musl][pthreads] Remove more vestiges of robust mutex support in stdio

This special 0x40000000 value was used to mark robust mutexes to be
cleaned up (ie, waiters awoken) at pthread_exit time. We no longer
have such a mechanism, and so no longer need the linked list of file
locks.

Change-Id: Id38ecc1908ad8b62289c4796978639f12c3a759a

[musl] Pull __wake and __wait out of pthread_impl.h

Another step towards disentangling pthreads.

Change-Id: I8ffbd594eaa62f1190ea6dd59b4b2aac8f8a02e6

[magenta] Initial commit