cmd/internal/obj/riscv: add tests for BGE/BGEU/BLT/BLTU

Add tests for BGE/BGEU/BLT/BLTU branch instructions. Also add pure Go variants
of these to ensure that the test data, Go and assembly all match up.

Change-Id: I84c68605e116a4e57f6c5c765bf0aaecab84b675
Reviewed-on: https://go-review.googlesource.com/c/go/+/271913


Trust: Joel Sing <joel@sing.id.au>
Reviewed-by: Quey-Liang Kao <s101062801@m101.nthu.edu.tw>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Go Bot <gobot@golang.org>