cmd/internal/obj/ppc64: fix PCALIGN on ppc64le

cmd/internal/obj/ppc64: fix PCALIGN on ppc64le

This fixes a potential issue with the previous implementation
of PCALIGN on ppc64. Previously PCALIGN was processed inside of
asmout and indicated the padding size by setting the value in
the optab, changing it back after the alignment instructions
were added. Now PCALIGN is processed outside of asmout, and optab
is not changed.

Change-Id: I8b0093a0e2b7e06176af27e05150d04ae2c55d60
Reviewed-on: https://go-review.googlesource.com/c/go/+/225198
Run-TryBot: Lynn Boger <laboger@linux.vnet.ibm.com>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Carlos Eduardo Seo <cseo@linux.vnet.ibm.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>