Add a testcase for PR ld/2404

PR ld/2404 was fixed without a testcase.  This patch to add one.

	PR ld/2404
	* ld-elf/pr2404.out: New file.
	* ld-elf/pr2404a.c: Likewise.
	* ld-elf/pr2404b.c: Likewise.

	* ld-elf/shared.exp (build_tests): Build libpr2404a.so and
	libpr2404b.a.
	(run_tests): Run pr2404.