cmd/compile: intrinsify Mul64 on riscv64

According to RISCV instruction set manual v2.2 Sec 6.1
MULHU followed by MUL will be fused into one multiply by microarchitecture

Benchstat on Hifive unmatched:
name          old time/op    new time/op    delta
Hash8Bytes       245ns ± 3%     186ns ± 4%  -23.99%  (p=0.000 n=10+10)
Hash320Bytes    1.94µs ± 1%    1.31µs ± 1%  -32.38%  (p=0.000 n=9+10)
Hash1K          5.84µs ± 0%    3.84µs ± 0%  -34.20%  (p=0.000 n=10+9)
Hash8K          45.3µs ± 0%    29.4µs ± 0%  -35.04%  (p=0.000 n=10+10)

name          old speed      new speed      delta
Hash8Bytes    32.7MB/s ± 3%  43.0MB/s ± 4%  +31.61%  (p=0.000 n=10+10)
Hash320Bytes   165MB/s ± 1%   244MB/s ± 1%  +47.88%  (p=0.000 n=9+10)
Hash1K         175MB/s ± 0%   266MB/s ± 0%  +51.98%  (p=0.000 n=10+9)
Hash8K         181MB/s ± 0%   279MB/s ± 0%  +53.94%  (p=0.000 n=10+10)

Change-Id: I3561495d02a4a0ad8578e9b9819bf0a4eaca5d12
Reviewed-on: https://go-review.googlesource.com/c/go/+/329970


Reviewed-by: Joel Sing <joel@sing.id.au>
Run-TryBot: Joel Sing <joel@sing.id.au>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Meng Zhuo <mzh@golangcn.org>