[NFC][AArch64] Get default features directly from ArchInfo and CpuInfo objects

This updates the AArch64's Target Parser and its uses to capture
information about default features directly from ArchInfo and CpuInfo
objects, instead of relying on an API function to access them
indirectly.

Reviewed By: tmatheson

Differential Revision: https://reviews.llvm.org/D142540