xxd.py: always write \n line endings

Previously, `open()` was used with the default option of `newline=None`,
which means that “any '\n' characters written are translated to the
system default line separator”. Now, `xxd.py` always writes `\n` line
endings. This eliminates the need for the .gitattributes file.

cmake/xxd.py

@@ -17,7 +17,7 @@ def main():
 
     binary_data = args.input.open("rb").read()
 
-    with args.output.open("w") as fout:
+    with args.output.open("w", newline="\n") as fout:
         fout.write("unsigned char {}[] = {{\n".format(varname))
         bytes_written = 0
         while bytes_written < len(binary_data):