cmd/internal/obj/riscv: avoid obj.Prog rewriting for address to register loads

Rather than rewriting the obj.Prog for a MOV pseudo-instruction targeting
an address to register load, generate the appropriate machine instruction
sequence directly.

Change-Id: I507195b6d24ff3253eadcc807ddbe27dca97d220
Reviewed-on: https://go-review.googlesource.com/c/go/+/344456


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