cmd/compile: add intrinsics for runtime/internal/math on MIPS64x

cmd/compile: add intrinsics for runtime/internal/math on MIPS64x

name              old time/op  new time/op  delta
MulUintptr/small  8.42ns ± 0%  5.93ns ± 0%  -29.66%  (p=0.000 n=9+10)
MulUintptr/large  11.1ns ± 0%   7.4ns ± 0%  -33.17%  (p=0.000 n=10+9)

Change-Id: I6659a886389660461fc2c90bd248243f6e7c29d5
Reviewed-on: https://go-review.googlesource.com/c/go/+/210897
Run-TryBot: Meng Zhuo <mengzhuo1203@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>