Fix -save-temp leaking lto files in /tmp

Fix -save-temp leaking lto files in /tmp

When linking with -flto and -save-temps, various
temporary files are created in /tmp.
The same happens when invoking the driver with @file
parameter, and using -L or -I options.

gcc:
2021-01-04  Bernd Edlinger  <bernd.edlinger@hotmail.de>

	* collect-utils.c (collect_execute): Check dumppfx.
	* collect2.c (maybe_run_lto_and_relink, do_link): Pass atsuffix
	to collect_execute.
	(do_link): Add new parameter atsuffix.
	(main): Handle -dumpdir option.  Skip one argument for
	-o, -isystem and -B options.
	* gcc.c (make_at_file): New helper function.
	(close_at_file): Use it.

gcc/testsuite:
2021-01-04  Bernd Edlinger  <bernd.edlinger@hotmail.de>

	* gcc.misc-tests/outputs.exp: Adjust testcase.