selftests: arm64: Add utility to set SVE vector lengths

vlset is a small utility for use in conjunction with tests like the sve-test
stress test which allows another executable to be invoked with a configured
SVE vector length.

Signed-off-by: Mark Brown <broonie@kernel.org>
Acked-by: Dave Martin <Dave.Martin@arm.com>
Acked-by: Shuah Khan <skhan@linuxfoundation.org>
Link: https://lore.kernel.org/r/20200819114837.51466-5-broonie@kernel.org


Signed-off-by: Will Deacon <will@kernel.org>