cmd/internal/obj/riscv: avoid obj.Prog rewriting for address to register loads
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 <4816dabf8db1bc6cac35b3a24cab2ff844b5b0c7@sing.id.au>
Reviewed-by: Cherry Mui <d62e63aa42ce272d7b6a5055d97e942b33a34679@google.com>

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