Make `get_externals` handle fetching platform-specific release artifacts

[savannahostrowski]

Right now, we are just using x86_64 LLVM binary for all Windows architectures in get_externals. We should use the aarch64 binary instead of relying on emulation, and update get_externals to handle that.

#141167 (comment)

Linked PRs

• GH-141362: Make get_externals handle fetching platform-specific release artifacts #142405