c-xcodeml.c triggers compilation error when passing NULL as va_list

[alexeedm]

When trying to compile the code on Arm, I'm getting the following errors:

"c-xcodeml.c", line 254: error: argument of type "void *" is incompatible with parameter of type "va_list"
          voutxTag(fp, indent, expr, tag, xattrFlag, attrFmt, NULL);
                                                              ^

"c-xcodeml.c", line 287: error: argument of type "void *" is incompatible with parameter of type "va_list"
              attrFmt, NULL);
                       ^

"c-xcodeml.c", line 306: error: argument of type "void *" is incompatible with parameter of type "va_list"
              attrFmt, NULL);
                       ^

By C standard va_list is not necessarily a pointer, so the cast is invalid.
Similar errors happen in the F-output-xcodeml.c file. I'll try to make a pull request with the fixes