cmd/compile: intrinsify Add64 on riscv64

cmd/compile: intrinsify Add64 on riscv64

According to RISCV instruction set manual v2.2 Sec 2.4, we can
implement overflowing check for unsigned addition cheaply using
SLTU instructions.

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

name                 old time/op    new time/op    delta
ScalarBaseMult/P256    1.93ms ± 1%    1.64ms ± 1%  -14.96%  (p=0.008 n=5+5)
ScalarBaseMult/P224    1.80ms ± 2%    1.53ms ± 1%  -14.89%  (p=0.008 n=5+5)
ScalarBaseMult/P384    6.15ms ± 2%    5.12ms ± 2%  -16.73%  (p=0.008 n=5+5)
ScalarBaseMult/P521    25.9ms ± 1%    22.3ms ± 2%  -13.78%  (p=0.008 n=5+5)
ScalarMult/P256        5.59ms ± 1%    4.49ms ± 2%  -19.79%  (p=0.008 n=5+5)
ScalarMult/P224        5.42ms ± 1%    4.33ms ± 1%  -20.01%  (p=0.008 n=5+5)
ScalarMult/P384        19.9ms ± 2%    16.3ms ± 1%  -18.15%  (p=0.008 n=5+5)
ScalarMult/P521        97.3ms ± 1%   100.7ms ± 0%   +3.48%  (p=0.008 n=5+5)

Change-Id: Ic4c82ced4b072a4a6575343fa9f29dd09b0cabc4
Reviewed-on: https://go-review.googlesource.com/c/go/+/420094
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Run-TryBot: Wayne Zuo <wdvxdr@golangcn.org>
Reviewed-by: Joel Sing <joel@sing.id.au>
TryBot-Result: Gopher Robot <gobot@golang.org>