emulators/i386-wine-devel: Use standard ports infrastructure for ARCH

Until now i386-wine-devel defined its own ARCH variable and manually
included Makefile.i386 for i386 systems and Makefile.inc for amd64
systems. This commit makes use of the standard ARCH variable and renames
Makefile.inc to Makefile.amd64 so that the proper Makefile.${ARCH} is
automatically included.
This requires conditional inclusion of bsd.ports.pre.mk and
bsd.ports.post.mk by emulators/wine-devel [1], master port for
i386-wine-devel: those files must be included only if they have not
already been included by i386-wine-devel.

Approved by: gerald (mentor, maintainer [1])