ld: Add a testcase for PR ld/22471

	PR ld/22471
	* testsuite/ld-elf/pr22471.t: New file.
	* testsuite/ld-elf/pr22471a.s: Likewise.
	* testsuite/ld-elf/pr22471b.s: Likewise.
	* testsuite/ld-elf/shared.exp (ASFLAGS): Define UNDERSCORE for
	underscore targets.
	Run PR ld/22471 tests.