cmd/compile: wire up math/bits.TrailingZeros intrinsics for loong64

cmd/compile: wire up math/bits.TrailingZeros 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                |
TrailingZeros     1.7240n ± 0%   0.8120n ± 0%  -52.90% (p=0.000 n=20)
TrailingZeros8    1.0530n ± 0%   0.8015n ± 0%  -23.88% (p=0.000 n=20)
TrailingZeros16    2.072n ± 0%    1.015n ± 0%  -51.01% (p=0.000 n=20)
TrailingZeros32   1.7160n ± 0%   0.8122n ± 0%  -52.67% (p=0.000 n=20)
TrailingZeros64   2.0060n ± 0%   0.8125n ± 0%  -59.50% (p=0.000 n=20)
geomean            1.669n        0.8470n       -49.25%

goos: linux
goarch: loong64
pkg: math/bits
cpu: Loongson-3A5000 @ 2500.00MHz
                |  bench.old   |              bench.new               |
                |    sec/op    |    sec/op     vs base                |
TrailingZeros     2.6275n ± 0%   0.9120n ± 0%  -65.29% (p=0.000 n=20)
TrailingZeros8     1.451n ± 0%    1.163n ± 0%  -19.85% (p=0.000 n=20)
TrailingZeros16    3.069n ± 0%    1.201n ± 0%  -60.87% (p=0.000 n=20)
TrailingZeros32   2.9060n ± 0%   0.9115n ± 0%  -68.63% (p=0.000 n=20)
TrailingZeros64   2.6305n ± 0%   0.9115n ± 0%  -65.35% (p=0.000 n=20)
geomean            2.456n         1.011n       -58.83%

This patch is a copy of CL 479498.
Co-authored-by: WANG Xuerui <git@xen0n.name>
Change-Id: I1a5b2114a844dc0d02c8e68f41ce2443ac3b5fda
Reviewed-on: https://go-review.googlesource.com/c/go/+/624356
Reviewed-by: abner chenc <chenguoqi@loongson.cn>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Keith Randall <khr@google.com>