Properly format pointer size independent CloudABI system calls.

CloudABI has approximately 50 system calls that do not depend on the
pointer size of the system. As the ABI is pretty compact, it takes
little effort to each truss(8) the formatting rules for these system
calls. Start off by formatting pointer size independent system calls.

Changes:

• Make it possible to include the CloudABI system call definitions in FreeBSD userspace builds. Add ${root}/sys to the truss(8) Makefile so we can pull in <compat/cloudabi/cloudabi_syscalldefs.h>.
• Refactoring: patch up amd64-cloudabi64.c to use the CLOUDABI_* constants instead of rolling our own table.
• Add table entries for all of the system calls.
• Add new generic formatting types (UInt, IntArray) that we'll be using to format unsigned integers and arrays of integers.
• Add CloudABI specific formatting types.

Approved by: jhb
Differential Revision: https://reviews.freebsd.org/D3836