cmd/compile, math: make math.{Abs,Copysign} intrinsics on loong64

cmd/compile, math: make math.{Abs,Copysign} intrinsics on loong64

goos: linux
goarch: loong64
pkg: math
cpu: Loongson-3A6000 @ 2500.00MHz
         │  old.bench   │              new.bench               │
         │    sec/op    │    sec/op     vs base                │
Copysign   1.9710n ± 0%   0.8006n ± 0%  -59.38% (p=0.000 n=10)
Abs        1.8745n ± 0%   0.8006n ± 0%  -57.29% (p=0.000 n=10)
geomean     1.922n        0.8006n       -58.35%

goos: linux
goarch: loong64
pkg: math
cpu: Loongson-3A5000 @ 2500.00MHz
         │  old.bench   │              new.bench               │
         │    sec/op    │    sec/op     vs base                │
Copysign   2.4020n ± 0%   0.9006n ± 0%  -62.51% (p=0.000 n=10)
Abs        2.4020n ± 0%   0.8005n ± 0%  -66.67% (p=0.000 n=10)
geomean     2.402n        0.8491n       -64.65%

Updates #59120.

Change-Id: Ic409e1f4d15ad15cb3568a5aaa100046e9302842
Reviewed-on: https://go-review.googlesource.com/c/go/+/580280
Reviewed-by: Qiqi Huang <huangqiqi@loongson.cn>
Reviewed-by: Meidan Li <limeidan@loongson.cn>
Reviewed-by: abner chenc <chenguoqi@loongson.cn>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: David Chase <drchase@google.com>