powerpc/powerpc64: Enforce natural alignment in memcpy

Summary:
POWER architecture CPUs (Book-S) require natural alignment for
cache-inhibited storage accesses. Since we can't know the caching model
for a page ahead of time, always enforce natural alignment in memcpy.
This fixes a SIGBUS in X with acceleration enabled on POWER9.

As part of this, revert r358672, it's no longer necessary with this fix.

Regression tested by alfredo.

Reviewed by: alfredo
Differential Revision: https://reviews.freebsd.org/D23969