btf: Change BTF_KIND_* macros to enums

Change BTF_KIND_* macros to enums so they are encoded in dwarf and
appear in vmlinux.h. This will make it easier for bpf programs
to use these constants without macro definitions.

Signed-off-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20210914223009.245307-1-yhs@fb.com