cmd/compile: intrinsify Sub64 on riscv64

cmd/compile: intrinsify Sub64 on riscv64

After this CL, the performance difference in crypto/elliptic
benchmarks on linux/riscv64 are:

name                 old time/op    new time/op    delta
ScalarBaseMult/P256    1.64ms ± 1%    1.60ms ± 1%   -2.36%  (p=0.008 n=5+5)
ScalarBaseMult/P224    1.53ms ± 1%    1.47ms ± 2%   -4.24%  (p=0.008 n=5+5)
ScalarBaseMult/P384    5.12ms ± 2%    5.03ms ± 2%     ~     (p=0.095 n=5+5)
ScalarBaseMult/P521    22.3ms ± 2%    13.8ms ± 1%  -37.89%  (p=0.008 n=5+5)
ScalarMult/P256        4.49ms ± 2%    4.26ms ± 2%   -5.13%  (p=0.008 n=5+5)
ScalarMult/P224        4.33ms ± 1%    4.09ms ± 1%   -5.59%  (p=0.008 n=5+5)
ScalarMult/P384        16.3ms ± 1%    15.5ms ± 2%   -4.78%  (p=0.008 n=5+5)
ScalarMult/P521         101ms ± 0%      47ms ± 2%  -53.36%  (p=0.008 n=5+5)

Change-Id: I31cf0506e27f9d85f576af1813630a19c20dda8a
Reviewed-on: https://go-review.googlesource.com/c/go/+/420095
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Joel Sing <joel@sing.id.au>
Reviewed-by: David Chase <drchase@google.com>
Run-TryBot: Wayne Zuo <wdvxdr@golangcn.org>
TryBot-Result: Gopher Robot <gobot@golang.org>