cmd/internal/obj/ppc64: add support for pcalign 32 on ppc64x

Previous PCALIGN support on ppc64x only accepted 8 and 16 byte
alignment since the default function alignment was 16. Now that
the function's alignment can be set to a larger value when needed,
PCALIGN can accept 32. When this happens then the function's
alignment will be changed to 32.

Test has been updated to recognized this new value.

Change-Id: If82c3cd50d7c686fcf8a9e819708b15660cdfa63
Reviewed-on: https://go-review.googlesource.com/c/go/+/227775


Run-TryBot: Lynn Boger <laboger@linux.vnet.ibm.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>