Update to ar_archive_writer 0.5
This updates `ar_archive_writer` to 0.5, which in turn was updated to match LLVM 20.1.8: <https://github.com/rust-lang/ar_archive_writer/pull/24>
As part of this, I refactored part of `SymbolWrapper.cpp` to pull common code that I was about to duplicate again into a new function.
NOTE: `ar_archive_writer` does include a breaking change where it no longer supports mangling C++ mangled names for Arm64EC. Since we don't need the mangled name (it's not the "exported name" which we're trying to load from the external dll), I'm setting the `import_name` when building for Arm64EC to prevent error when failing to mangle.
r? `@bjorn3`
7aef4bec4b