Remove full barrier from the amd64 atomic_load_acq_*(). Strong

Remove full barrier from the amd64 atomic_load_acq_*().  Strong
ordering semantic of x86 CPUs makes only the compiler barrier
neccessary to give the acquire behaviour.

Existing implementation ensured sequentially consistent semantic for
load_acq, making much stronger guarantee than required by standard's
definition of the load acquire.  Consumers which depend on the barrier
are believed to be identified and already fixed to use proper
operations.

Noted by:	alc (long time ago)
Reviewed by:	alc, bde
Tested by:	pho
Sponsored by:	The FreeBSD Foundation
MFC after:	2 weeks