cmd/compiler,cmd/go,sync: add internal {LoadAcq,StoreRel}64 on ppc64
Add an internal atomic intrinsic for load with acquire semantics
(extending LoadAcq to 64b) and add LoadAcquintptr for internal
use within the sync package. For other arches, this remaps to the
appropriate atomic.Load{,64} intrinsic which should not alter code
generation.
Similarly, add StoreRel{uintptr,64} for consistency, and inline.
Finally, add an exception to allow sync to directly use the
runtime/internal/atomic package which avoids more convoluted
workarounds (contributed by Lynn Boger).
In an extreme example, sync.(*Pool).pin consumes 20% of wall time
during fmt tests. This is reduced to 5% on ppc64le/power9.