btf: add BTF_KIND_FUNC traversal function.

btf: add BTF_KIND_FUNC traversal function.

The patch adds a traversal function to traverse all BTF_KIND_FUNC nodes
with a callback function. Used for .BTF.ext section content creation.

gcc/ChangeLog:

	* btfout.cc (output_btf_func_types): Use FOR_EACH_VEC_ELT.
	(traverse_btf_func_types): Define function.
	* ctfc.h (funcs_traverse_callback): Typedef for function
	prototype.
	(traverse_btf_func_types): Add prototype.

(cherry picked from commit 69a3ce49bda929e1ffbc1fc1123f5f2485ec944d)