kselftest/arm64: signal: Support signal frames with SVE register data

A signal frame with SVE may validly either be a bare struct sve_context or
a struct sve_context followed by vector length dependent register data.
Support either in the generic helpers for the signal tests, and while we're
at it validate the SVE vector length reported.

Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20210819134245.13935-3-broonie@kernel.org


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