Based on patch from https://gcc.gnu.org/bugzilla/show_bug.cgi?id=56030
Blocking bug https://gcc.gnu.org/bugzilla/show_bug.cgi?id=54040 has been fixed

--- gcc-4.9.2/gcc/ada/gcc-interface/Makefile.in~	2014-12-27 17:19:06.000000000 +0000
+++ gcc-4.9.2/gcc/ada/gcc-interface/Makefile.in	2014-12-27 18:11:33.908691021 +0000
@@ -355,6 +355,14 @@
   else
     ifeq ($(strip $(MULTISUBDIR)),/x32)
       target_cpu:=x32
+    else
+      ifeq ($(strip $(MULTISUBDIR)),/64)
+        target_cpu:=x86_64
+      else
+        ifneq ($(findstring gnux32,$(target_os)),)
+          target_cpu:=x32
+        endif
+      endif
     endif
   endif
 endif