For ELF linker, a common symbol isn't a definition. When we decide if a
symbol should be re-exported, we should check if the symbol isn't
undefined, not if it is a definition.
bfd/
PR ld/18914
* elflink.c (elf_link_add_object_symbols): Don't re-export a
symbol if it isn't undefined.
ld/testsuite/
PR ld/18914
* ld-elf/exclude.exp: Also check exclude_common.
* ld-elf/exclude2.s: Add exclude_common.