lto/96385 - avoid unused global UNDEFs in debug objects

Unused global UNDEFs can have side-effects in some circumstances so
the following patch avoids them by treating them the same as other
to be discarded DEFs - make them local.

2020-08-03  Richard Biener  <rguenther@suse.de>

	PR lto/96385
libiberty/
	* simple-object-elf.c
	(simple_object_elf_copy_lto_debug_sections): Localize global
	UNDEFs and reuse the prevailing name.