cmd/compile: optimize math/bits.OnesCount{16,32,64} implementation on loong64

cmd/compile: optimize math/bits.OnesCount{16,32,64} implementation on loong64

Use Loong64's LSX instruction VPCNT to implement math/bits.OnesCount{16,32,64}
and make it intrinsic.

Benchmark results on loongson 3A5000 and 3A6000 machines:

goos: linux
goarch: loong64
pkg: math/bits
cpu: Loongson-3A5000-HV @ 2500.00MHz
            |   bench.old   |   bench.new                          |
            |    sec/op     |    sec/op       vs base               |
OnesCount      4.413n ± 0%     1.401n ± 0%   -68.25% (p=0.000 n=10)
OnesCount8     1.364n ± 0%     1.363n ± 0%         ~ (p=0.130 n=10)
OnesCount16    2.112n ± 0%     1.534n ± 0%   -27.37% (p=0.000 n=10)
OnesCount32    4.533n ± 0%     1.529n ± 0%   -66.27% (p=0.000 n=10)
OnesCount64    4.565n ± 0%     1.531n ± 1%   -66.46% (p=0.000 n=10)
geomean        3.048n          1.470n        -51.78%

goos: linux
goarch: loong64
pkg: math/bits
cpu: Loongson-3A6000 @ 2500.00MHz
            |   bench.old   |   bench.new                          |
            |    sec/op     |    sec/op       vs base              |
OnesCount       3.553n ± 0%     1.201n ± 0%  -66.20% (p=0.000 n=10)
OnesCount8     0.8021n ± 0%    0.8004n ± 0%   -0.21% (p=0.000 n=10)
OnesCount16     1.216n ± 0%     1.000n ± 0%  -17.76% (p=0.000 n=10)
OnesCount32     3.006n ± 0%     1.035n ± 0%  -65.57% (p=0.000 n=10)
OnesCount64     3.503n ± 0%     1.035n ± 0%  -70.45% (p=0.000 n=10)
geomean         2.053n          1.006n       -51.01%

Change-Id: I07a5b8da2bb48711b896387ec7625145804affc8
Reviewed-on: https://go-review.googlesource.com/c/go/+/620978
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Meidan Li <limeidan@loongson.cn>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>