kselftest/arm64: Don't probe the current VL for unsupported vector types

kselftest/arm64: Don't probe the current VL for unsupported vector types

The vec-syscfg selftest verifies that setting the VL of the currently
tested vector type does not disrupt the VL of the other vector type. To do
this it records the current vector length for each type but neglects to
guard this with a check for that vector type actually being supported. Add
one, using a helper function which we also update all the other instances
of this pattern.

Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20231218-kselftest-arm64-vec-syscfg-rdvl-v1-1-0ac22d47e81f@kernel.org
Signed-off-by: Will Deacon <will@kernel.org>