MFC: r241374

Add an unified macro to deny ability from the compiler to reorder
instruction loads/stores at its will.
The macro __compiler_membar() is currently supported for both gcc and
clang, but kernel compilation will fail otherwise.

Reviewed by:	bde, kib
Discussed with:	dim, theraven
MFC: r251782, r251783, r253994

- Remove conflicting macros from SPARC64's atomic(9) header.
- Add MD (for now) atomic_store_acq_<type>() and use it in pmap_activate()
  to get the semantics when setting the PMAP right.