bug with -native true and MLton_bug

[Stephen Weeks]

> The only use of making it
> 	char	*MLton_bug_msg = "...";
> instead of
> 	char	MLton_bug_msg[] = "...";
> is that in the former case the string is not writable and is in the text space
> (so multiple running instances of the program will share that memory).  In the
> latter the string is writable and resides in the data space.
> All of this is irrelevant for a short string and more than compensated for
> by the extra indirection.

OK.  I made the change.  Thanks.