Properly format pointer size independent CloudABI system calls.

[ed]

Summary

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.

Test Plan

truss(8) has been run on cloudlibc's unit testing binary. This yields
calls for all of the system calls that are part of the ABI. The
formatting of all entries have been checked.

[jhb]

I think this looks fine. I'm planning to re-sort the enum of argument types in a bit so that there are simple scalars (Int, UInt, etc.) followed by symbolic scalars (e.g. flag fields) followed by objects referenced by pointer. I'm happy to keep the ABI specific types in separate sections as well as you have done here.