cmd/internal/obj/riscv: support subtraction with a constant
Allow SUB and SUBW to be specified with a constant, which are mapped
to ADDI and ADDIW with negated values.
Change-Id: I7dc55692febc81ea87393b0a3a7d23a43c30313b
Reviewed-on: https://go-review.googlesource.com/c/go/+/538915
Run-TryBot: Joel Sing <joel@sing.id.au> Reviewed-by: Cherry Mui <cherryyz@google.com> Reviewed-by: M Zhuo <mzh@golangcn.org>
TryBot-Result: Gopher Robot <gobot@golang.org> Reviewed-by: Heschi Kreinick <heschi@google.com> Reviewed-by: Mark Ryan <markdryan@rivosinc.com> Reviewed-by: Wang Yaduo <wangyaduo@linux.alibaba.com> Reviewed-by: Mauri de Souza Meneguzzo <mauri870@gmail.com>