kselftest/arm64: signal: Check SVE signal frame shows expected vector length

As a basic check that the SVE signal frame is being set up correctly
verify that the vector length in the signal frame is the vector length
that the process has.

Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20210819134245.13935-4-broonie@kernel.org


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