btf: add 'extern' linkage for variables [PR106773]

btf: add 'extern' linkage for variables [PR106773]

Add support for the 'extern' linkage value for BTF_KIND_VAR records,
which is used for variables declared as extern in the source file.

This also fixes a bug with BTF generation for extern variables which
have both a non-defining declaration and a defining declaration in the
same CU.

	PR target/106773

gcc/

	* btfout.cc (btf_collect_datasec): Mark extern variables as such.
	(btf_dvd_emit_preprocess_cb): Skip non-defining extern variable decl
	if there is a defining decl for the same variable.
	(btf_asm_varent): Accomodate 'extern' linkage.

gcc/testsuite/

	* gcc.dg/debug/btf/btf-variables-4.c: New test.
	* gcc.dg/debug/btf/btf-variables-5.c: New test.

include/

	* btf.h (enum btf_var_linkage): New.
	(struct btf_var): Update comment to note 'extern' linkage.