Check whether 32-bit x86 is really 64-bit

Fixes configure on OS X 10.6

Originally committed as revision 20134 to svn://svn.ffmpeg.org/ffmpeg/trunk

configure

@@ -1817,20 +1817,17 @@ case "$arch" in
         enable fast_64bit
         spic=$shared
     ;;
-    i386|i486|i586|i686|i86pc|BePC)
+    i386|i486|i586|i686|i86pc|BePC|x86_64|amd64)
         arch="x86"
         subarch="x86_32"
         enable fast_unaligned
-    ;;
-    x86_64|amd64)
-        arch="x86"
-        subarch="x86_32"
-        enable cmov
-        enable fast_cmov
-        enable fast_unaligned
         check_cc <<EOF && enable fast_64bit && subarch="x86_64" && spic=$shared
         int test[sizeof(char*) - 7];
 EOF
+        if test "$subarch" = "x86_64"; then
+            enable cmov
+            enable fast_cmov
+        fi
     ;;
     *)
         arch="unknown"