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

This commit is the analogous for i386-wine of r528249 that concerns
emulators/i386-wine-devel.

Until now i386-wine 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 bsd.ports.pre.mk and bsd.ports.post.mk
by emulators/wine [1], master port for i386-wine: those files must be
included only if they have not already been included by i386-wine.

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