cmd/internal/obj/ppc64: add support for PCALIGN 32

This adds support support for the PCALIGN value 32. When this
directive occurs code will be aligned to 32 bytes unless
too many NOPs are needed, and then will fall back to 16
byte alignment.

On Linux the function's alignment is promoted from 16 to 32
in functions where PCALIGN 32 appears. On AIX the function's
alignment is left at 16 due to complexity with modifying its
alignment, which means code will be aligned to at least 16,
possibly 32 at times, which is still good.

Test was updated to accept new value.

Change-Id: I28e72d5f30ca472ed9ba736ddeabfea192d11797
Reviewed-on: https://go-review.googlesource.com/c/go/+/228258


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