cmd/compile: wire up bits.Reverse intrinsics for loong64

cmd/compile: wire up bits.Reverse intrinsics for loong64

Micro-benchmark results on Loongson 3A5000 and 3A6000:

goos: linux
goarch: loong64
pkg: math/bits
cpu: Loongson-3A6000 @ 2500.00MHz
          |  CL 624576   |               this CL                |
          |    sec/op    |    sec/op     vs base                |
Reverse     2.8130n ± 0%   0.8008n ± 0%  -71.53% (p=0.000 n=20)
Reverse8    0.7014n ± 0%   0.4040n ± 0%  -42.40% (p=0.000 n=20)
Reverse16   1.2975n ± 0%   0.6632n ± 1%  -48.89% (p=0.000 n=20)
Reverse32   2.7520n ± 0%   0.4042n ± 0%  -85.31% (p=0.000 n=20)
Reverse64   2.8970n ± 0%   0.4041n ± 0%  -86.05% (p=0.000 n=20)
geomean      1.828n        0.5116n       -72.01%

goos: linux
goarch: loong64
pkg: math/bits
cpu: Loongson-3A5000 @ 2500.00MHz
          |  CL 624576   |               this CL                |
          |    sec/op    |    sec/op     vs base                |
Reverse     4.0050n ± 0%   0.8011n ± 0%  -80.00% (p=0.000 n=20)
Reverse8    0.8010n ± 0%   0.5210n ± 1%  -34.96% (p=0.000 n=20)
Reverse16   1.6160n ± 0%   0.6008n ± 0%  -62.82% (p=0.000 n=20)
Reverse32   3.8550n ± 0%   0.5179n ± 0%  -86.57% (p=0.000 n=20)
Reverse64   3.8050n ± 0%   0.5177n ± 0%  -86.40% (p=0.000 n=20)
geomean      2.378n        0.5828n       -75.49%

Updates #59120

This patch is a copy of CL 483656.
Co-authored-by: WANG Xuerui <git@xen0n.name>
Change-Id: I98681091763279279c8404bd0295785f13ea1c8e
Reviewed-on: https://go-review.googlesource.com/c/go/+/624276
Reviewed-by: abner chenc <chenguoqi@loongson.cn>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: David Chase <drchase@google.com>