Provide a default DeviceArchVariant

The empty string is no longer a valid arm64 arch variant.

Change-Id: I61468ba09f9fd629e9ebd3915e266490416f65d6
diff --git a/common/variable.go b/common/variable.go
index 350bfda..de31324 100644
--- a/common/variable.go
+++ b/common/variable.go
@@ -98,6 +98,7 @@
 		HostSecondaryArch:          stringPtr("x86"),
 		DeviceName:                 stringPtr("flounder"),
 		DeviceArch:                 stringPtr("arm64"),
+		DeviceArchVariant:          stringPtr("armv8-a"),
 		DeviceCpuVariant:           stringPtr("denver64"),
 		DeviceAbi:                  &[]string{"arm64-v8a"},
 		DeviceUsesClang:            boolPtr(true),