RTTI makes the binaries bigger, exceptions make life harder for
optimisers. Neither are actually used.
This doesn't preclude anything #including snmalloc.h with RTTI enabled,
it just disables it in the shim libraries and tests.
The FreeBSD libc builds were already doing this with no ill effect.
The encoding in the top bits for the size class did not respect kernel
pointers. Using an intptr_t means, we can use a signed shift to
maintain the kernel pointers.
