tools/bpf: expose functions btf_ext__* as API functions

tools/bpf: expose functions btf_ext__* as API functions

The following set of functions, which manipulates .BTF.ext
section, are exposed as API functions:
  . btf_ext__new
  . btf_ext__free
  . btf_ext__reloc_func_info
  . btf_ext__reloc_line_info
  . btf_ext__func_info_rec_size
  . btf_ext__line_info_rec_size

These functions are useful for JIT based bpf codegen, e.g.,
bcc, to manipulate in-memory .BTF.ext sections.

The signature of function btf_ext__reloc_func_info()
is also changed to be the same as its definition in btf.c.

Acked-by: Martin KaFai Lau <kafai@fb.com>
Signed-off-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>