Specify hidden visibility for global data symbol declarations

See https://gcc.godbolt.org/z/rTse83vPz for an example of how this affects code generation.

Even without this patch the GOT indirection can be eliminated by the linker, but unless LTO is used the generated code will still be suboptimal as it can only substitute instructions byte-for-byte, not rewrite entire blocks of code. This allows for improved code generation directly by the compiler.