cmd/compile: lowered MulUintptr 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

name              old time/op  new time/op  delta
MulUintptr/small  11.2ns ±24%   9.2ns ± 0%  -17.54%  (p=0.000 n=10+9)
MulUintptr/large  15.9ns ± 0%  10.9ns ± 0%  -31.55%  (p=0.000 n=8+8)

Change-Id: I3d152218f83948cbc5c576bda29dc86e9b4206ee
Reviewed-on: https://go-review.googlesource.com/c/go/+/338753


Trust: Meng Zhuo <mzh@golangcn.org>
Reviewed-by: Joel Sing <joel@sing.id.au>