makefile: Fix contrib/djgpp/libc/crt0 build (previous commit broken)

The previous commit broke "make clean": it would delete the entire
	contrib/djgpp/libc/crt0/
directory, including the _main.c source file in it...

Now we're using
	contrib/djgpp/libc/crt0/obj/<target>/_main.o
instead of
	contrib/djgpp/libc/crt0/_main.o
i.e. a target-specific obj/ subdirectory, like everything else, which means
that "make clean" will only delete the obj/ directory now, not the source
file.