cmd/internal/obj/riscv: avoid obj.Prog rewriting for immediate splitting

Rather than rewriting the obj.Prog for a immediate instructions that need
splitting, generate the appropriate machine instruction sequence directly.

Change-Id: Ie90f0e2a98f97a29281e445c4c3b0c47b793ef4d
Reviewed-on: https://go-review.googlesource.com/c/go/+/344453


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