makesyscalls: deprecate cpp other than includes

Warn that C preprocessor directives in the config file are deprecated.
They are unsound and support has a number of potential pitfalls. They
should be replaced by compile-time generation of files plus an overlay
framework to allow things like per-arch variation.

Reviewed by: kevans
Sponsored by: DARPA, AFRL
Pull Request: https://github.com/freebsd/freebsd-src/pull/1575