Only enable MLGO for ARM64 ThinLTO targets

The MLGO model we embed in the Android Clang is trained on ARM64
ThinLTO artefacts. Applying the model across architectures does not
necessarily translate to performance improvement. Therefore only enable
the optimisation on ARM64 ThinLTO for now.

Test: presubmit
Bug: 293827654
Change-Id: Ie64a65c11191cf700a463637a0746c8470d3aa3c