Make sure that undefined symbols added to the linker command line via the -u option...

Make sure that undefined symbols added to the linker command line via the -u option appear in the output executable, if they have not been resolved.

	PR 22319
bfd	* elflink.c (elf_link_output_extsym): Keep global undefined
	symbols if they have been marked as needed.

ld	* testsuite/ld-elf/pr22310.s: New test source file.
	* testsuite/ld-elf/pr22310.d: New test driver.
	* testsuite/ld-mmix/undef-3.d: Update expected output from readelf.