Rid libdummy.a from lto.exp

libdummy.a is used to make run_cc_link_tests compile objects and
archive them.  libdummy.a isn't used.  What we're really doing is
preventing a final link.  So do that directly.

	* lib/ld-lib.exp (run_ld_link_tests): Stop after assembling objects
	if binfile is empty.
	(run_ld_link_exec_tests, run_cc_link_tests): Likewise.  Tidy
	status checks.
	* ld-plugin/lto.exp: Don't use libdummy.a trick to compile objects.
	Instead use an empty output file.
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