cmd/internal/obj/riscv: absorb rewriteMOV into instruction generation
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 <4816dabf8db1bc6cac35b3a24cab2ff844b5b0c7@sing.id.au>
Reviewed-by: Cherry Mui <d62e63aa42ce272d7b6a5055d97e942b33a34679@google.com>

nawawi/go,golang/go,CAFxX/go,nawawi/go,golang/go,CAFxX/go,CAFxX/go,nawawi/go,nawawi/go,cloudflare/go,cloudflare/go,cloudflare/go,golang/go,golang/go,golang/go,cloudflare/go,nawawi/go,CAFxX/go,CAFxX/go,cloudflare/go,cloudflare/go,CAFxX/go,CAFxX/go,golang/go,golang/go,nawawi/go,CAFxX/go,nawawi/go,cloudflare/go,cloudflare/go,nawawi/go,golang/go