testsuite: Handle --save-temps in schedule-cleanups

Some tests use --save-temps, but schedule-cleanups strictly matches
-save-temps, so we leave many temporary files after validation.
Instead of fixing every offending testcase, it's simpler and
future-proof to make schedule-cleanups handle both --save-temps and
-save-temps.

2020-04-22  Christophe Lyon  <christophe.lyon@linaro.org>

	gcc/testsuite/
	* lib/gcc-dg.exp (schedule-cleanups): Accept --save-temps.