Fix the wait6(2) on 32bit architectures and for the compat32, by using
the right type for the argument in syscalls.master. Also fix the
posix_fallocate(2) and posix_fadvise(2) compat32 syscalls on the
architectures which require padding of the 64bit argument.

Noted and reviewed by: jhb
Pointy hat to: kib
MFC after: 1 week