Details:
- Switched the small block allocator (sba), as defined in bli_sba.c and
bli_apool.c, to static initialization of its internal mutex. Did a
similar thing for the packing block allocator (pba), which appears as
global_membrk in bli_membrk.c.
- Commented out bli_membrk_init_mutex() and bli_membrk_finalize_mutex()
to ensure they won't be used in the future.
- In bli_thrcomm_pthreads.c and .h, removed old, commented-out cpp
blocks guarded by BLIS_USE_PTHREAD_MUTEX.
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