syscalls: annotate args pointing to long, pointer, or time_t

Add _Contains_ annotations indicating that the data pointed to by a
pointer argument contains types that vary between FreeBSD ABIs. The
supported set is long (including size_t), pointer (including
intptr_t), and time_t. The first two vary between 32- and 64-bit
ABIs. The laste betwen i386 and everything else.

These will be used to detect which syscalls require handling on
particular ABIs.

Reviewed by: kevans, kib