cmd/asm/internal,cmd/internal/obj/ppc64: add alignment directive to asm for ppc64x

This adds support for an alignment directive that can be used
within Go asm to indicate preferred code alignment for ppc64x.
This is intended to be used with loops to improve
performance.

This change only adds the directive and aligns the code based
on it. Follow up changes will modify asm functions for
ppc64x that benefit from preferred alignment.

Fixes #14935

Here is one example of the improvement in memmove when the
directive is used on the loops in the code:

Memmove/64      8.74ns ± 0%    8.64ns ± 0%   -1.19%  (p=0.000 n=8+8)
Memmove/128     11.5ns ± 0%    11.0ns ± 0%   -4.35%  (p=0.000 n=8+8)
Memmove/256     23.0ns ± 0%    15.3ns ± 0%  -33.48%  (p=0.000 n=8+8)
Memmove/512     31.7ns ± 0%    31.8ns ± 0%   +0.32%  (p=0.000 n=8+8)
Memmove/1024    52.3ns ± 0%    43.9ns ± 0%  -16.10%  (p=0.000 n=8+8)
Memmove/2048    93.2ns ± 0%    76.2ns ± 0%  -18.24%  (p=0.000 n=8+8)
Memmove/4096     174ns ± 0%     141ns ± 0%  -18.97%  (p=0.000 n=8+8)

Change-Id: I200d77e923dd5d78c22fe3f8eb142a8fbaff57bf
Reviewed-on: https://go-review.googlesource.com/c/144218
Run-TryBot: Lynn Boger <laboger@linux.vnet.ibm.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>

src/cmd/internal/obj/util.go

@@ -535,6 +535,7 @@ var Anames = []string{
 	"FUNCDATA",
 	"JMP",
 	"NOP",
+	"PCALIGN",
 	"PCDATA",
 	"RET",
 	"GETCALLERPC",