The backend emits `.loh` directives for arm64_32 as well. Our pass
already handles 32-bit pointer loads correctly (there was an extraneous
sanity check for 8-byte pointer sizes, I removed that here), so we can
enable them for all arm64 subtargets, including our upcoming arm64e
support.
Moving it away from the arm64 `TargetInfo` class will let us enable it
more easily for arm64_32 and the soon-to-be-added arm64e target as well.
This is the NFC part of #148964