kselftest/arm64: Size sycall-abi buffers for the actual maximum VL

Our ABI opts to provide future proofing by defining a much larger
SVE_VQ_MAX than the architecture actually supports.  Since we use
this define to control the size of our vector data buffers this results
in a lot of overhead when we initialise which can be a very noticable
problem in emulation, we fill buffers that are orders of magnitude
larger than we will ever actually use even with virtual platforms that
provide the full range of architecturally supported vector lengths.

Define and use the actual architecture maximum to mitigate this.

Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20230810-arm64-syscall-abi-perf-v1-1-6a0d7656359c@kernel.org


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