bpf: btf: Add BTF_KFUNCS_START/END macro pair

This macro pair is functionally equivalent to BTF_SET8_START/END, except
with BTF_SET8_KFUNCS flag set in the btf_id_set8 flags field. The next
commit will codemod all kfunc set8s to this new variant such that all
kfuncs are tagged as such in .BTF_ids section.

Signed-off-by: Daniel Xu <dxu@dxuuu.xyz>
Link: https://lore.kernel.org/r/d536c57c7c2af428686853cc7396b7a44faa53b7.1706491398.git.dxu@dxuuu.xyz


Signed-off-by: Alexei Starovoitov <ast@kernel.org>