Make ABI tests generate .out files.

If you rerun "make check" in a tree where some tests have already been
run, it will rerun ABI tests because those do not create an output
file.

This patch changes those tests to create .out files so they only get
rerun if the dependencies (on the ABI baselines and the generated
.symlist files) indicate they should be rerun.

Tested x86_64.

	* Makerules (check-abi-%): Change target to
	$(objpfx)check-abi-%.out.
	(check-abi target): Update dependencies.
	(check-abi-pattern variable): Redirect output of diff to $@.
	(check-abi variable): Likewise.
	* elf/Makefile (check-abi): Update dependencies.