[TSan][Darwin] Fix shadow mapping for iOS simulator on Apple Silicon

With the introduction of Apple Silicon `defined(__aarch64__)` is not a
reliable way to check for the platform anymore.

We want to use the "normal" `Mapping48AddressSpace` mapping everywhere
except devices, including the iOS simulators on AS.

Relevant revisions:
https://reviews.llvm.org/D35147
https://reviews.llvm.org/D86377
https://reviews.llvm.org/D107743
https://reviews.llvm.org/D107888

Differential Revision: https://reviews.llvm.org/D115843