Cirrus-CI: add a manually triggered arm64 task

Reviewed by:	imp
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D31953

(cherry picked from commit 8cba2003e8)

.cirrus.yml

@@ -14,7 +14,16 @@ env:
   CIRRUS_CLONE_DEPTH: 1
 
 task:
-  name: World and kernel amd64 build and boot smoke test
+  matrix:
+  - name: World and kernel amd64 build and boot smoke test
+    env:
+      TARGET: amd64
+      TARGET_ARCH: amd64
+  - name: World and kernel arm64 build and boot smoke test
+    trigger_type: manual
+    env:
+      TARGET: arm64
+      TARGET_ARCH: aarch64
   timeout_in: 120m
   install_script:
   - sh .cirrus-ci/pkg-install.sh qemu llvm11