fix threaded PPC build, refactor new semaphore code

fix threaded PPC build, refactor new semaphore code

 Refactor the recent semaphore code into nicer shape.

 * No point in having these functions as inline functions.

 * Move shared logic to os-common.c, and conditionalize it on
   CANNOT_USE_POSIX_SEM_T, which platform specific headers can define if they
   implement their own versions. (Darwin now, Windows in the future.)

 * PPC needs genesis/thread.h in assembly, not thread.h.