[ld, testsuite] Restrict dl1*main and dl6*main tests on platforms which have libdl support

ld/
	* testsuite/lib/ld-lib.exp (check_libdl_available): New function.
	* testsuite/ld-elf/shared.exp (run_tests): Split tests which require
	dlopen support into "dlopen_run_tests".  These tests include dl1*main
	and dl6*main.
	(dlopen_run_tests): New and only run it when check_libdl_available
	returns true.  XFAIL on *-*-netbsdelf*.