cmd/compile: wire up Bswap/ReverseBytes intrinsics for loong64

cmd/compile: wire up Bswap/ReverseBytes intrinsics for loong64

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

goos: linux
goarch: loong64
pkg: math/bits
cpu: Loongson-3A6000 @ 2500.00MHz
               |  bench.old   |              bench.new               |
               |    sec/op    |    sec/op     vs base                |
ReverseBytes     2.0020n ± 0%   0.4040n ± 0%  -79.82% (p=0.000 n=20)
ReverseBytes16   0.8866n ± 1%   0.8007n ± 0%   -9.69% (p=0.000 n=20)
ReverseBytes32   1.2195n ± 0%   0.8007n ± 0%  -34.34% (p=0.000 n=20)
ReverseBytes64   2.0705n ± 0%   0.8008n ± 0%  -61.32% (p=0.000 n=20)
geomean           1.455n        0.6749n       -53.62%

goos: linux
goarch: loong64
pkg: math/bits
cpu: Loongson-3A5000 @ 2500.00MHz
               |  bench.old   |              bench.new               |
               |    sec/op    |    sec/op     vs base                |
ReverseBytes     2.8040n ± 0%   0.5205n ± 0%  -81.44% (p=0.000 n=20)
ReverseBytes16   0.7066n ± 0%   0.8011n ± 0%  +13.37% (p=0.000 n=20)
ReverseBytes32   1.5500n ± 0%   0.8010n ± 0%  -48.32% (p=0.000 n=20)
ReverseBytes64   2.7665n ± 0%   0.8010n ± 0%  -71.05% (p=0.000 n=20)
geomean           1.707n        0.7192n       -57.87%

Updates #59120

This patch is a copy of CL 483357.
Co-authored-by: WANG Xuerui <git@xen0n.name>
Change-Id: If355354cd031533df91991fcc3392e5a6c314295
Reviewed-on: https://go-review.googlesource.com/c/go/+/624576
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: abner chenc <chenguoqi@loongson.cn>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Carlos Amedee <carlos@golang.org>