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.
David Faust
2bce22e88e