Bug 1540082 - add a `toolchain-arm64-build` docker image; r=nalexander

We need this image for building clang on machines with arm64
sysroots.  (Note that this image *is* a linux x86-64 image, just with
some arm64 cross-compilation packages available.)

Differential Revision: https://phabricator.services.mozilla.com/D28404

--HG--
extra : moz-landing-system : lando

taskcluster/ci/docker-image/kind.yml

@@ -162,6 +162,10 @@ jobs:
     definition: debian-build
     args:
       ARCH: arm64
+  toolchain-arm64-build:
+    symbol: I(toolchain-arm64)
+    parent: debian9-arm64-build
+    definition: toolchain-build
   android-build:
     symbol: I(agb)
     parent: debian9-base