* libc/include/signal.h (SIG_IGN, SIG_DFL, SIG_ERR): Change
to use _sig_func_ptr type casted constants.
(_sig_func_ptr): Typedef moved to sys/signal.h.
* libc/include/sys/signal.h (_sig_func_ptr): Typedef added.
For __rtems, use POSIX definition, otherwise default to ANSI.
* libc/sys/linux/sys/signal.h (_sig_func_ptr): Typedef added.