cmd/compile: avoid the use of XOR for boolean equality on riscv64

cmd/compile: avoid the use of XOR for boolean equality on riscv64

The use of SEQZ/SNEZ and SUB allows for other optimisations to be utilised,
particularly absorption into branch equality conditions.

Change-Id: I74e7d6a07a8decc1bdb651660c322bcc6eb6a10a
Reviewed-on: https://go-review.googlesource.com/c/go/+/428216
Run-TryBot: Joel Sing <joel@sing.id.au>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Meng Zhuo <mzh@golangcn.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>