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.
Stop io/ftwtest deleting its own output.

If you rerun "make check" in a tree where some tests have already been
run, it will rerun io/ftwtest-sh because that test uses ftwtest.out,
the same name to which output is redirected, as its internal temporary
file, and then removes it on exit.

Clearly tests should not be removing the files to which their output
is redirected like that.  This patch changes the script to use a
different file as its internal temporary file, so the actual output
referenced in the makefile isn't removed.

Tested x86_64.

	* io/ftwtest-sh (testout): Change to $tmp/ftwtest-tmp.out.