BPF: make __builtin_btf_type_id() return 64bit int

Linux kernel recently added support for kernel modules
  https://lore.kernel.org/bpf/20201110011932.3201430-5-andrii@kernel.org/

In such cases, a type id in the kernel needs to be presented
as (btf id for modules, btf type id for this module).
Change __builtin_btf_type_id() to return 64bit value
so libbpf can do the above encoding.

Differential Revision: https://reviews.llvm.org/D91489