cmd/internal/obj/riscv: implement RV64I integer computational instructions

Add support for assembling RV64I integer computational instructions.

Based on the riscv-go port.

Updates #27532

Integer Computational Instructions (RV64I)

Change-Id: I1a082b3901c997da309d737d081f57ea2821bc62
Reviewed-on: https://go-review.googlesource.com/c/go/+/196838


Reviewed-by: Cherry Zhang <cherryyz@google.com>
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>