cmd/compile: optimize small zeroing/moving on riscv64

Optimize small (s <= 32) zeroing/moving operations on riscv64.
Avoid generating unaligned memory accesses.

The code is almost one to one translation of the corresponding
mips64 rules with additional rule for s=32.

Change-Id: I753b0b8e53cb9efcf43c8080cab90f3d03539fb8
Reviewed-on: https://go-review.googlesource.com/c/go/+/266217


Reviewed-by: Joel Sing <joel@sing.id.au>
Reviewed-by: Cherry Zhang <cherryyz@google.com>