cmd/internal/obj/riscv: absorb rewriteMOV into instruction generation

Replace the now poorly named rewriteMOV function with a markRelocs function,
absorbing the MOV validation into the instruction generation code.

Change-Id: I6ae94553f7f300862c40310343ef0be8f44a4b0f
Reviewed-on: https://go-review.googlesource.com/c/go/+/344459


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