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

[savannahostrowski]

This PR adds support for downloading platform-specific LLVM binaries (AMD64, ARM64) instead of using a single binary with emulation (see https://github.com/python/cpython-bin-deps/releases/tag/llvm-21.1.4.0 for release artifacts). Since this won't naturally trigger jit.yml, you can see a successful run here, including print statements to show that we are in fact downloading the correct binary 😄.

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

[zooba]

Can we instead based this on %PreferredToolArchitecture% (will be x86, x64 or ARM64), and possibly update the Exec task in the build files to pass it through? It's set as a build-time variable ($(PreferredToolArchitecture)) but if it's overridden (from the current default, x86) then likely it's set as an environment variable.

This is the same setting we use for all "things that run during build", and is used by MSVC when selecting which version of its own LLVM to use.

(If I'm misunderstanding why we need to select the architecture here now and it's because of the target architecture, not the build machine architecture, then same suggested but use $(Platform) instead.)

[savannahostrowski]

I'm not sure PreferredToolArchitecture would give us the right answer here - we need LLVM that can actually run on the build host, not LLVM that matches a tool preference. For example, if someone set PreferredToolArchitecture=x86 on an ARM64 machine, we'd still want ARM64 LLVM since that's what can execute natively. Does that make sense, or am I missing a scenario?

[savannahostrowski]

Will remove later :D