genmatch: fixup get_out_file

get_out_file did not follow the coding conventions (mixing three-space
and two-space indentation, missing linebreak before function name).

Take that as an excuse to reimplement it in a more terse manner and
rename as 'choose_output', which is hopefully more descriptive.

gcc/ChangeLog:

	* genmatch.cc (get_out_file): Make static and rename to ...
	(choose_output): ... this. Reimplement. Update all uses ...
	(decision_tree::gen): ... here and ...
	(main): ... here.