Disabling ssp directory

Hi

RTEMS has its own version of the stack protection functions.

To avoid potential conflicts, I'd like to disable ssp/ from the build.

There is currently no conditional mechanism to disable it.

Would adding another newlib_cflags like HAVE_SSP be ok?

It seems like the easiest solution and is aligns with what has been done in
other cases where the target provides a function also in newlib.

Thanks.

--joel