cmd/internal/obj/riscv: fix and enable test in short mode

cmd/internal/obj/riscv: fix and enable test in short mode

The branch test only takes a few seconds so enable it in short mode. Also fix a
typo that currently prevents the code from compiling.

Change-Id: I25fc646b002a66c042c61fb1f56d5efa8e47903c
Reviewed-on: https://go-review.googlesource.com/c/go/+/356249
Trust: Joel Sing <joel@sing.id.au>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Run-TryBot: Cherry Mui <cherryyz@google.com>
TryBot-Result: Go Bot <gobot@golang.org>