[BPF] Add support for floats and doubles

Some BPF programs compiled on s390 fail to load, because s390
arch-specific linux headers contain float and double types. At the
moment there is no BTF_KIND for floats and doubles, so the release
version of LLVM ends up emitting type id 0 for them, which the
in-kernel verifier does not accept.

Introduce support for such types to libbpf by representing them using
the new BTF_KIND_FLOAT.

Reviewed By: yonghong-song

Differential Revision: https://reviews.llvm.org/D83289