AArch64 linux.thread-reset fails on non-SVE hardware

When adding SVE signal context support (https://github.com/DynamoRIO/dynamorio/pull/6725), `linux.thread-reset` started failing intermittently on _non-SVE_ hardware.

To replicate the failure, build in DEBUG or RELEASE mode and run the unit test. It will fail almost all of the time in RELEASE mode and will fail most of the time in DEBUG mode. Attempts to debug using GDB or just with `-debug -loglevel` will result in the test passing almost all of the time.