Cast the pointer assigned to ss_sp to char *.

Cast the pointer assigned to ss_sp to char *.

FreeBSD versions older than 11.0 use char * as the type of ss_sp in
stack_t instead of the standards-defined void *.  C++ allows a char *
pointer to be converted to a void *, so it is safe to cast the return
value of xmalloc to char * if ss_sp is either a char * or void *.
Just always use the cast to char * since that is less ugly than having
to add a special case.

gdb/ChangeLog:

	* main.c (setup_alternate_signal_stack): Cast to char *.