pthread: Refactor semaphore code

The internal semaphore list code is moved to a specific file,
sem_routine.c, and the internal usage is simplified to only two
functions (one to insert a new semaphore and one to remove it
from the internal list).  There is no need to expose the
internal locking, neither how the semaphore mapping is implemented.

No functional or semantic change is expected, tested on
x86_64-linux-gnu.