kselftest/arm64: Fix enumeration of systems without 128 bit SME for SSVE+ZA

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 in the newly
added ssve_za_regs test.

Fixes: bc69da5f ("kselftest/arm64: Verify simultaneous SSVE and ZA context generation")
Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20230131-arm64-kselftest-sig-sme-no-128-v1-2-d47c13dc8e1e@kernel.org


Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>