This adds math/bits intrinsics for OnesCount on arm64.

name         old time/op  new time/op  delta
OnesCount    3.81ns ± 0%  1.60ns ± 0%  -57.96%  (p=0.000 n=7+8)
OnesCount8   1.60ns ± 0%  1.60ns ± 0%     ~     (all equal)
OnesCount16  2.41ns ± 0%  1.60ns ± 0%  -33.61%  (p=0.000 n=8+8)
OnesCount32  4.17ns ± 0%  1.60ns ± 0%  -61.58%  (p=0.000 n=8+8)
OnesCount64  3.80ns ± 0%  1.60ns ± 0%  -57.84%  (p=0.000 n=8+8)

Update #18616

Conflicts:
	src/cmd/compile/internal/gc/asm_test.go

Change-Id: I63ac2f63acafdb1f60656ab8a56be0b326eec5cb
Reviewed-on: https://go-review.googlesource.com/90835
Run-TryBot: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Cherry Zhang <cherryyz@google.com>