[dev.power64] runtime: fix atomicor8 for power64x

Power64 servers do not currently support sub-word size atomic
memory access, so atomicor8 uses word size atomic access.
However, previously atomicor8 made no attempt to align this
access, resulting in errors.  Fix this by aligning the pointer
to a word boundary and shifting the value appropriately.
Since atomicor8 is used in GC, add a test to runtime·check to
make sure this doesn't break in the future.

This also fixes an incorrect branch label, an incorrectly
sized argument move, and adds argument names to help go vet.

LGTM=rsc
R=rsc, dave
CC=golang-codereviews
https://codereview.appspot.com/165820043