all: clean up addition of constants in riscv64 assembly

all: clean up addition of constants in riscv64 assembly

Use ADD with constants, instead of ADDI. Also use SUB with a positive constant
rather than ADD with a negative constant. The resulting assembly is still the
same.

Change-Id: Ife10bf5ae4122e525f0e7d41b5e463e748236a9c
Reviewed-on: https://go-review.googlesource.com/c/go/+/540136
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: M Zhuo <mzh@golangcn.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Mark Ryan <markdryan@rivosinc.com>
Reviewed-by: Heschi Kreinick <heschi@google.com>
Run-TryBot: Joel Sing <joel@sing.id.au>