Fix arch in snapshot DepsMutator.

Suffixes attached to snapshot modules are based on the main architecture
(e.g. "arm64" for aosp_cf_x86_64_phone-userdebug), not the architecture
of arch variants.

Bug: 184588077
Bug: 184695066
Test: Run unit tests using:
Test: source build/envsetup.sh
Test: m -j nothing
Change-Id: Id8d002d45864a6fb12d642b60011e7a9eaf29e07
diff --git a/cc/snapshot_prebuilt.go b/cc/snapshot_prebuilt.go
index 5f80d56..af05102 100644
--- a/cc/snapshot_prebuilt.go
+++ b/cc/snapshot_prebuilt.go
@@ -337,7 +337,8 @@
 		for _, name := range names {
 			snapshotMap[name] = name +
 				getSnapshotNameSuffix(snapshotSuffix+moduleSuffix,
-					s.baseSnapshot.version(), ctx.Arch().ArchType.Name)
+					s.baseSnapshot.version(),
+					ctx.DeviceConfig().Arches()[0].ArchType.String())
 		}
 		return snapshotMap
 	}