[lldb][AArch64] Handle different default vector length in SVE testing

This test previously ran on QEMU or A64FX both of which can/do have
512 bit SVE by default.

Graviton 3 has 256 bit SVE so the first part of the test failed.

To fix this, probe the supported vector lengths before starting
the test. The first check will use the default vector length and
the rest use either 256 or 128 bit.

Therefore this test will be skipped on a machine with only 128 bit SVE.

Reviewed By: omjavaid

Differential Revision: https://reviews.llvm.org/D154208