cmd/asm,cmd/internal/obj/riscv: add more error tests for riscv64 assembly
Add more error tests for riscv64 assembly. Also avoid a panic when one of
these error conditions is hit.
Change-Id: If5d913894facbd67f7d014eab745da77c8c66ab0
Reviewed-on: https://go-review.googlesource.com/c/go/+/344228
Trust: Joel Sing <joel@sing.id.au>
Run-TryBot: Joel Sing <joel@sing.id.au>
TryBot-Result: Go Bot <gobot@golang.org> Reviewed-by: Meng Zhuo <mzh@golangcn.org>