MingW: Don't add suffix for nul device

MingW: Don't add suffix for nul device

This patch fixes an issue where on systems that are
HAVE_TARGET_EXECUTABLE_SUFFIX the driver calls convert_filename in order to
add the suffix to the filename.  However while it excludes `-` it doesn't
exclude the null device.  This patches changes the check to exclude anything
that is not a file by calling not_actual_file_p instead.

This also fixes a bug in not_actual_file_p which was accidentally testing
a the global variable output_file instead of the supplied argument.  This
hasn't been an issue so far because because not_actual_file_p was only used
on output_file till now.

This fixes the adding of an extension to the nul device which is against
the recommendations on msdn[0] and makes it harder for the next tool in line
to detect it.

Bootstrapped Regtested on x86_64-w64-mingw32 and no issues.
Did do a bootstrap on x86_64-pc-linux-gnu but no regtest as it's not a
HAVE_TARGET_EXECUTABLE_SUFFIX system.

[0] https://docs.microsoft.com/en-us/windows/win32/fileio/naming-a-file

gcc/ChangeLog:

	PR driver/97574
	* gcc.c (convert_filename): Don't add suffix to things that are
	not files.
	(not_actual_file_p): Use supplied argument.