Don't re-export common symbols

Don't re-export common symbols

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.