Fix Windows-target testing in mi_gdb_file_cmd

Similar to my recent fix for gdb_file_cmd, mi_gdb_file_cmd also runs
into problems when GCC has created foo.exe given "-o foo".

Apply exactly the same fix there as in gdb_file_cmd.  This allows many
more tests to succeed for Windows target that previously fell over.

2020-11-18  Joseph Myers  <joseph@codesourcery.com>

	* lib/mi-support.exp (mi_gdb_file_cmd): Check for case where
	$arg.exe exists but $arg does not.