Rename mock arch to fake.

This fits better with the definitions of test doubles as the arch
pretends to be an arch rather than just providing stubs.

Change-Id: I07856e1c04e2e97f6df4daf2b4420fa07763848f
diff --git a/BUILD.gn b/BUILD.gn
index 7962f70..8258888 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -27,7 +27,7 @@
   testonly = true
 
   deps = [
-    "//src:unit_tests($mock_toolchain)",
+    "//src:unit_tests($fake_toolchain)",
   ]
 }