arm/machine: create generic-arm64 machine

Create a generic Arm64 system for creating a SystemReady file system.
Due to it being as generic as possible, the image created
by this should run on any hardware that already has the requisite
firmware present.

Change-Id: Ie0d71f91145311e90d228bc403e653a7ef059838
Signed-off-by: Jon Mason <jon.mason@arm.com>

.gitlab-ci.yml

@@ -76,6 +76,9 @@ gem5-arm64:
 gem5-arm64-xen:
   extends: .build
 
+generic-arm64:
+  extends: .build
+
 juno:
   extends: .build