Unify redundant llvm cache keys

[alandefreitas]

At some point we started using the llvm-archive-basename as the cache key for LLVM and at some other point the llvm-archive-basename started to include extra information about the compiler.

This led to redundant cache keys because these libraries should have the same ABI (meaning we shouldn't need the compiler or the compiler version in the cache key). At the same time, they could be somewhat conflicting because they would have a different ABI in a different OS (meaning we might want the OS in the cache key).