cmd/compile: improve subtraction of constants on riscv64

Convert subtraction of a constant into an ADDI with a negative immediate,
where possible.

Change-Id: Ie8d54b7538f0012e5f898abea233b2957fe31899
Reviewed-on: https://go-review.googlesource.com/c/go/+/221679


Reviewed-by: Cherry Zhang <cherryyz@google.com>