btf: emit linkage information in BTF_KIND_FUNC entries

btf: emit linkage information in BTF_KIND_FUNC entries

The kernel bpftool expects BTF_KIND_FUNC entries in BTF to include an
annotation reflecting the linkage of functions (static, global).  For
whatever reason they abuse the `vlen' field of the BTF_KIND_FUNC entry
instead of adding a variable-part to the record like it is done with
other entry kinds.

This patch makes GCC to include this linkage info in BTF_KIND_FUNC
entries.

Tested in bpf-unknown-none target.

gcc/ChangeLog:

	PR debug/106263
	* ctfc.h (struct ctf_dtdef): Add field linkage.
	* ctfc.cc (ctf_add_function): Set ctti_linkage.
	* dwarf2ctf.cc (gen_ctf_function_type): Pass a linkage for
	function types and subprograms.
	* btfout.cc (btf_asm_func_type): Emit linkage information for the
	function.
	(btf_dtd_emit_preprocess_cb): Propagate the linkage information
	for functions.

gcc/testsuite/ChangeLog:

	PR debug/106263
	* gcc.dg/debug/btf/btf-function-4.c: New test.
	* gcc.dg/debug/btf/btf-function-5.c: Likewise.