sfxge(4): move empty efsys definitions to EFX headers

Move empty definitions for platform-specific annotations from efsys.h
to EFX headers.

Submitted by: Martin Harvey <mharvey at solarflare.com>
Submitted by: Andrew Lee <alee at solarflare.com>
Sponsored by: Solarflare Communications, Inc.
Differential Revision: https://reviews.freebsd.org/D18248