kselftest/arm64: Fix enumeration of systems without 128 bit SME

kselftest/arm64: Fix enumeration of systems without 128 bit SME

The current signal handling tests for SME do not account for the fact that
unlike SVE all SME vector lengths are optional so we can't guarantee that
we will encounter the minimum possible VL, they will hang enumerating VLs
on such systems. Abort enumeration when we find the lowest VL.

Fixes: 4963aeb35a9e ("kselftest/arm64: signal: Add SME signal handling tests")
Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20230131-arm64-kselftest-sig-sme-no-128-v1-1-d47c13dc8e1e@kernel.org
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>