Update hurd/intr-msg.c to be more portable

Update hurd/intr-msg.c to be more portable

Summary of the changes:
- Introduce BAD_TYPECHECK from MiG to make it simpler to do type
  checking.
- Replace int type with mach_msg_type_t. This assumes that
  mach_msg_type_t is always the same size as int which is not true for
  x86_64.
- Calculate the size and align using PTR_ALIGN_UP, which is a bit
  cleaner and similar to what we do elsewhere.
- Define mach_msg_type_t to check using designated initializers.
Message-Id: <ZFMvrIkvoCSxqB/C@jupiter.tail36e24.ts.net>