scripts/build aarch64-clang target: fix clang ver

Apparently, there's no clang-3.8 for Ubuntu Trusty arm64. The latest
available version is 3.6, let's try to use it.

https://travis-ci.org/kolyshkin/criu/builds/173168198Signed-off-by: Kir Kolyshkin <kir@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>

scripts/build/Makefile

@@ -21,6 +21,7 @@ alpine:
 alpine-clang: DB_ARGS=--build-arg CC=clang
 alpine-clang: alpine
 armv7hf-clang: armv7hf
+aarch64-clang: DB_ARGS=--build-arg CC=clang-3.6
 aarch64-clang: aarch64
 ppc64le-clang: ppc64le
 x86_64-clang: x86_64