1.0.41.8: x86/x86-64: Weaken read and write barrier operations.

1.0.41.8: x86/x86-64: Weaken read and write barrier operations.

  * It turns out that the read-barrier operation is only needed on
Pentium Pro systems for SMP operation.  This never worked right,
so there's no need to penalize other systems to support this.

  * It turns out that the write-barrier operation is only needed
on OOSTORE systems for SMP operation.  Thsi never worked right, so
there's no need to penalize other systems to support this.

  * It turns out that x86-64 systems are neither Pentium Pro nor
OOSTORE.

  * This breaks down if the barriers were required for device I/O
instead of SMP operation.  Should anyone actually need such
barriers, adding them to the existing framework separately should
be simple enough.