[clang] [llvm] [BPF] Fix linking issues in static map initializers (PR #91310)

[Nick Zavaritsky via llvm-commits]

mejedi wrote:

@yonghong-song @dwblaikie 
> Is there some test coverage that shows that unreferenced variables/functions aren't included in the output?

Test added.

https://github.com/llvm/llvm-project/pull/91310