internal/bytealg: add assembly implementation of Count/CountString for riscv64

Simple single-byte loop count for now, to be further improved in future
CLs.

Benchmark on linux/riscv64 (HiFive Unleashed):

name               old time/op    new time/op     delta
CountSingle/10-4      190ns ± 1%      145ns ± 1%  -23.66%  (p=0.000 n=10+9)
CountSingle/32-4      422ns ± 1%      268ns ± 0%  -36.43%  (p=0.000 n=10+7)
CountSingle/4K-4     43.3µs ± 0%     23.8µs ± 0%  -45.09%  (p=0.000 n=8+10)
CountSingle/4M-4     54.2ms ± 1%     33.3ms ± 1%  -38.48%  (p=0.000 n=10+10)
CountSingle/64M-4     1.52s ± 1%      1.20s ± 1%  -21.20%  (p=0.000 n=9+9)

name               old speed      new speed       delta
CountSingle/10-4   52.7MB/s ± 1%   69.1MB/s ± 1%  +31.03%  (p=0.000 n=10+9)
CountSingle/32-4   75.9MB/s ± 1%  119.5MB/s ± 0%  +57.34%  (p=0.000 n=10+8)
CountSingle/4K-4   94.6MB/s ± 0%  172.2MB/s ± 0%  +82.10%  (p=0.000 n=8+10)
CountSingle/4M-4   77.4MB/s ± 1%  125.8MB/s ± 1%  +62.54%  (p=0.000 n=10+10)
CountSingle/64M-4  44.2MB/s ± 1%   56.1MB/s ± 1%  +26.91%  (p=0.000 n=9+9)

Change-Id: I2a6bd50d22d5f598517bb3c5a50066c54280cac5
Reviewed-on: https://go-review.googlesource.com/c/go/+/263541


Trust: Tobias Klauser <tobias.klauser@gmail.com>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Joel Sing <joel@sing.id.au>