test/codegen: add riscv64 codegen for arithmetic tests
Codify the current riscv64 code generation for various subtract from
constant and addition/subtraction tests.
Change-Id: I54ad923280a0578a338bc4431fa5bdc0644c4729
Reviewed-on: https://go-review.googlesource.com/c/go/+/652316 Reviewed-by: Meng Zhuo <mengzhuo1203@gmail.com> Reviewed-by: Cherry Mui <cherryyz@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> Reviewed-by: Mark Ryan <markdryan@rivosinc.com> Reviewed-by: David Chase <drchase@google.com>