gen_syscall_header.py: script to generate macros

This header could be maintained by hand since there are no inputs
and it only changes if the generating script is modified, but given
the choice to maintain 800-ish lines of extremely repetitive C
preprocessor code, or 100-ish lines of Python, the choice is pretty
clear.

Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>