Don't complain undefined weak dynamic reference

Don't complain undefined weak dynamic reference

When undefined non-weak references in IR objects are optimized out
by LTO, we can have weak dynamic referencs to symbols marked with
bfd_link_hash_undefined.  We shouldn't complain such undefined weak
dynamic references.

bfd/

	PR ld/24486
	* elflink.c (elf_link_output_extsym): Don't complain undefined
	weak dynamic reference.

ld/

	PR ld/24486
	* testsuite/ld-plugin/lto.exp: Run PR ld/24486 tests.
	* testsuite/ld-plugin/pr24486a.c: New file.
	* testsuite/ld-plugin/pr24486b.c: Likewise.
	* testsuite/ld-plugin/pr24486c.c: Likewise.