cmd/internal/obj/riscv: prevent panics on bad branches

Syntactically incorrect branches, such as

BEQ	X5, X6, $1
BEQ	X5, X6, 31(X10)

cause the assembler to panic, which they shouldn't really do.  It's
better for the user to see a normal error, as reported for other
syntax errors in riscv64 assembly.  The panics also prevent us
from writing negative tests for these sorts of errors.

Here we fix the issue by ensuring we generate a normal error instead
of panicking when the user provides an invalid branch target.  We
also add a couple of negative tests.

Change-Id: I1da568999a75097484b61a01d418f5d4be3e04fa
Reviewed-on: https://go-review.googlesource.com/c/go/+/637316
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Joel Sing <joel@sing.id.au>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Meng Zhuo <mengzhuo1203@gmail.com>

0 files changed, 0 insertions, 0 deletions