AArch64 SVE: Add 128 bit vector length tests to CI

[AssadHashmi]

The current SVE tests (aarch64-precommit (ubuntu-20-arm64-sve)) run on hardware which has a 256 bit vector length (VL) as a thread default, the maximum on that hardware. The fix for https://github.com/DynamoRIO/dynamorio/issues/6760 exposed lack of test coverage based on different VLs.

SVE hardware and SVE support in the Linux kernel enables users to select VLs shorter than the maximum. We can use the same hardware and the prctl(2) syscall to set VL to 128 to defend changes in VL sensitive code paths. This would be in addition to the existing tests using VL of 256 bits.