cmd/asm, cmd/internal/obj/riscv: fix branch pseudo-instructions

Pseudo branch instructions BGT, BGTU, BLE, and BLEU implemented In
CL 226397 were translated inconsistently compared to other ones due
to the inversion of registers. For instance, while "BLT a, b" generates
"jump if a < b", "BLE a, b" generates "jump if b <= a."

This CL fixes the translation in the assembler and the tests.

Change-Id: Ia757be73e848734ca5b3a790e081f7c4f98c30f2
Reviewed-on: https://go-review.googlesource.com/c/go/+/271911


Reviewed-by: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Joel Sing <joel@sing.id.au>
Run-TryBot: Joel Sing <joel@sing.id.au>