Add wait4() back to the set of <asm-generic/unistd.h> syscalls.

The initial pass at the generic ABI assumed that wait4() could be
easily expressed using waitid().  Although it's true that wait4()
can be built on waitid(), it's awkward enough that it makes more
sense to continue to include wait4 in the generic syscall ABI.

Since there is already a deprecated wait4 in the ABI, this change
converts that wait4 into old_wait, and puts wait4 in the next
available slot for new supported syscalls, after the platform-specific
syscalls at number 260.

Signed-off-by: Chris Metcalf <cmetcalf@tilera.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>