configure: Detect aarch64 toolchains automatically

Change-Id: Icafda81dbc3323fa0afdba5f1c8758e812cc592a
diff --git a/build/make/configure.sh b/build/make/configure.sh
index b4d6bc9..b305d33 100644
--- a/build/make/configure.sh
+++ b/build/make/configure.sh
@@ -644,6 +644,9 @@
 
     # detect tgt_isa
     case "$gcctarget" in
+      aarch64*)
+        tgt_isa=arm64
+        ;;
       armv6*)
         tgt_isa=armv6
         ;;