Mark C globals with small code model

We have new option in clang (https://github.com/llvm/llvm-project/pull/124834)
to mark globals to be allocated in non-large sections. We can mark all globals
that are referenced from hardcoded assembly (which implicitly references globals
assuming they are in non-large sections) with this attribute to avoid running
into problems when dav1d is built with -mcmodel=medium with clang.