bug with -native true and MLton_bug

[Henry Cejtin]

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.