ego now builds and is used.

This needed lots of refactoring to ego --- not all platforms have ego descr
files, and ego will just crash if you invoke it without one. I think originally
it was never intended that these platforms would be used at -O2 or above.

Plats now only specify the ego descr file if they have one.

plat/linux68k/descr

@@ -26,6 +26,7 @@ var C_LIB={PLATFORMDIR}/libc-ansi.a
 var CC_ALIGN=-Vr
 var OLD_C_LIB={C_LIB}
 var MACHOPT_F=-ml10
+var EGO_PLAT_FLAGS=-M{EM}/share/ack/ego/{ARCH}.descr
 
 # Override the setting in fe so that files compiled for linux68k can see
 # the platform-specific headers.