platform autodetect: accept amd64 as a synonym for x86_64

Thanks to James Cloos <cloos at jhcloos dot com> for the tip.

Change-Id: If377cc084dd7c16a4f51191a2aa0d83e7117ebec

build/make/configure.sh

@@ -501,7 +501,7 @@ process_common_toolchain() {
 
         # detect tgt_isa
         case "$gcctarget" in
-            *x86_64*)
+            *x86_64*|*amd64*)
                 tgt_isa=x86_64
                 ;;
             *i[3456]86*)