add xchg on POWER with xlC __fetch_and_swap
add plasma_atomic_st_*_release macros
minor: code aesthetics and doc
plasma_atomic_fetch_add_*
plasma_spin_pause() using _Asm_hint(_HINT_PAUSE)

plasma_spin_pause() using _Asm_hint(_HINT_PAUSE) with HP aCC compiler
default atomic_thread_fence() to seq_cst
prefer gcc,clang atomic_thread_fence() intrinsics
upcast unsigned char prior to bitshift
plasma_spin_pause() - use _mm_pause() intrinsic

plasma_spin_pause() - use _mm_pause() intrinsic on x86 in gcc,clang
(when available)
plasma_spin_load() - encapsulate use of volatile
add enum _mm_hint for use in __builtin_prefetch()

add enum _mm_hint from _mm_prefetch() for use in __builtin_prefetch()
(can use _MM_HINT_* instead of integer literals to specify locality)
use _MM_HINT_* in calls to __builtin_prefetch()