testsuite: rename force_conventional_output

The procedure force_conventional_output_for is a bit misnomed, what it
primarily does is to set the required options for the corresponding
test. So rename the proc to set_required_options_for and also rename the
participating variable accordingly.

gcc/testsuite/ChangeLog:

	* lib/gcc-dg.exp: Rename gcc_force_conventional_output to
	gcc_set_required_options.
	* lib/target-supports.exp: Rename force_conventional_output_for
	to set_required_options_for.
	* lib/scanasm.exp: Adjust callers.
	* lib/scanrtl.exp: Same.