Cast via intptr_t when converting integers to pointers

ptrdiff_t is not guaranteed to be the same size as a pointer. Use
inptr_t instead as a more standards-compliant way to convert integers
to pointers. This fixes compiler warnings when targeting CHERI/Morello.
10 jobs for int-to-ptr-casts in 7 minutes and 18 seconds (queued for 1 minute and 10 seconds)