[clang] [Darwin] Add reverse mappings for aarch64/aarch64_32 to darwin arch names

These are mapped in MachO::getMachOArchName already, but were missing
in ToolChain::getDefaultUniversalArchName.

Having these reverse mapped here fixes weird inconsistencies like
-dumpmachine showing a target triple like "aarch64-apple-darwin",
while "clang -target aarch64-apple-darwin" didn't use to work (ended
up mapped as unknown-apple-ios).

Differential Revision: https://reviews.llvm.org/D79117