This patch will fix the m68k-elf builds on AMD64 machines.  But don't use it,
because it is unclear if the assertion or the code calling do_SUBST is at
fault.  You have been warned.

diff -ur gcc-3.4.6.orig/gcc/combine.c gcc-3.4.6/gcc/combine.c
--- gcc-3.4.6.orig/gcc/combine.c	2005-08-08 14:41:04.000000000 -0400
+++ gcc-3.4.6/gcc/combine.c	2006-04-22 20:35:58.000000000 -0400
@@ -442,9 +442,11 @@
     {
       /* Sanity check that we're replacing oldval with a CONST_INT
 	 that is a valid sign-extension for the original mode.  */
-      if (INTVAL (newval) != trunc_int_for_mode (INTVAL (newval),
+    /*
+     if (INTVAL (newval) != trunc_int_for_mode (INTVAL (newval),
 						 GET_MODE (oldval)))
 	abort ();
+    */
 
       /* Replacing the operand of a SUBREG or a ZERO_EXTEND with a
 	 CONST_INT is not valid, because after the replacement, the