Add "ld -r" tests for PR ld/15323

Add "ld -r" tests for PR ld/15323

Weak defined function is turned into non-weak defined function by
"ld -r -flto" with GCC 5 due to a GCC 5 regression:

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67548

Add "ld -r" tests for PR ld/15323 to make sure that any linker change
won't introduce linker regression for PR ld/15323.

	* ld-plugin/lto.exp (lto_link_tests): Add a "ld -r" test for
	PR ld/15323.
	(lto_run_tests): Add a "ld -r" test for PR ld/15323.