cmd/compile,internal/cpu,runtime: intrinsify math/bits.OnesCount on riscv64

cmd/compile,internal/cpu,runtime: intrinsify math/bits.OnesCount on riscv64

For riscv64/rva22u64 and above, we can intrinsify math/bits.OnesCount
using the CPOP/CPOPW machine instructions. Since the native Go
implementation of OnesCount is relatively expensive, it is also
worth emitting a check for Zbb support when compiled for rva20u64.

On a Banana Pi F3, with GORISCV64=rva22u64:

              │     oc.1     │                oc.2                 │
              │    sec/op    │   sec/op     vs base                │
OnesCount-8     16.930n ± 0%   4.389n ± 0%  -74.08% (p=0.000 n=10)
OnesCount8-8     5.642n ± 0%   5.016n ± 0%  -11.10% (p=0.000 n=10)
OnesCount16-8    9.404n ± 0%   5.015n ± 0%  -46.67% (p=0.000 n=10)
OnesCount32-8   13.165n ± 0%   4.388n ± 0%  -66.67% (p=0.000 n=10)
OnesCount64-8   16.300n ± 0%   4.388n ± 0%  -73.08% (p=0.000 n=10)
geomean          11.40n        4.629n       -59.40%

On a Banana Pi F3, compiled with GORISCV64=rva20u64 and with Zbb
detection enabled:

              │     oc.3     │                oc.4                 │
              │    sec/op    │   sec/op     vs base                │
OnesCount-8     16.930n ± 0%   5.643n ± 0%  -66.67% (p=0.000 n=10)
OnesCount8-8     5.642n ± 0%   5.642n ± 0%        ~ (p=0.447 n=10)
OnesCount16-8   10.030n ± 0%   6.896n ± 0%  -31.25% (p=0.000 n=10)
OnesCount32-8   13.170n ± 0%   5.642n ± 0%  -57.16% (p=0.000 n=10)
OnesCount64-8   16.300n ± 0%   5.642n ± 0%  -65.39% (p=0.000 n=10)
geomean          11.55n        5.873n       -49.16%

On a Banana Pi F3, compiled with GORISCV64=rva20u64 but with Zbb
detection disabled:

              │    oc.3     │                oc.5                 │
              │   sec/op    │   sec/op     vs base                │
OnesCount-8     16.93n ± 0%   29.47n ± 0%  +74.07% (p=0.000 n=10)
OnesCount8-8    5.642n ± 0%   5.643n ± 0%        ~ (p=0.191 n=10)
OnesCount16-8   10.03n ± 0%   15.05n ± 0%  +50.05% (p=0.000 n=10)
OnesCount32-8   13.17n ± 0%   18.18n ± 0%  +38.04% (p=0.000 n=10)
OnesCount64-8   16.30n ± 0%   21.94n ± 0%  +34.60% (p=0.000 n=10)
geomean         11.55n        15.84n       +37.16%

For hardware without Zbb, this adds ~5ns overhead, while for hardware
with Zbb we achieve a performance gain up of up to 11ns. It is worth
noting that OnesCount8 is cheap enough that it is preferable to stick
with the generic version in this case.

Change-Id: Id657e40e0dd1b1ab8cc0fe0f8a68df4c9f2d7da5
Reviewed-on: https://go-review.googlesource.com/c/go/+/660856
Reviewed-by: Carlos Amedee <carlos@golang.org>
Reviewed-by: Meng Zhuo <mengzhuo1203@gmail.com>
Reviewed-by: Mark Ryan <markdryan@rivosinc.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>