PR ld/21900: MIPS: Fix relocation processing with undefined symbols

Currently, when `mips_elf_calculate_relocation' is asked to relocate an
undefined symbol, it reports an error or a warning and immediately
returns without performing the relocation.  This is fine if the link
fails, but if unresolved_syms_in_objects == RM_GENERATE_WARNING, the
link will continue and output some unrelocated code, which is a
regression from commit e7e2196d ("MIPS/BFD: Correctly report
undefined relocations").

Fix this by continuing after calling the `undefined_symbol' hook unless
this is an error condition.

	bfd/
	PR ld/21900
	* elfxx-mips.c (mips_elf_calculate_relocation): Only return
	after calling `undefined_symbol' hook if this is an error
	condition.  Assume the value of 0 for the symbol requested
	otherwise.

	ld/
	PR ld/21900
	* testsuite/ld-mips-elf/undefined-warn.d: New test.
	* testsuite/ld-mips-elf/undefined.s: Add padding at the end.
	* testsuite/ld-mips-elf/mips-elf.exp: Run the new test.