BUILD: Add port specific makefile for AmigaOS4.

This moves the AmigaOS4 specific packaging rules from the global
ports.mk to a port specific makefile in the AmigaOS subdirectory of
the SDL backend (used for AmigaOS).

Previously, port specific makefiles were only used for cross compiling
builds and thus had a single entry in the cross compiling section of
the configure. Since AmigaOS4 is a desktop system supporting native
build, this required a second entry for when the host system is detected
as ppc-amigaos to support native builds.

However, currently this does break packaging of cross compiled builds
for Win32, OSX and Unix when done on AmigaOS4... but this is not likely
and has limited impact. To fix this, default _port_mk lines would need
to be added to the sections of the cross compiling switch for mingw32
etc. to override the AmigaOS4 OS setting of _port_mk.