mingw: Support building with older gcc versions

mingw: Support building with older gcc versions

The $@ argument to the compiler is causing
only a warning in some gcc versions but an
error in others. In any case, $@ was never
necessary so remove it completely, just like
the rules in x-mingw32 where the object file
gets named after the source file.

This fixes both warnings and errors about
sym-mingw32.o appearing in the command line
unnecessarily.

The -nostdlib flag is required along with -r
for older gcc versions that don't apply it
automatically with -r, resulting in main
functions erroneously entering a partial link.

Signed-off-by: Jonathan Yong <10walls@gmail.com>
gcc/ChangeLog:

	* config/i386/x-mingw32-utf8: Remove extrataneous $@