[test spec] Compare against the target cpu, not the current

Currently, a host-side test cannot target an arm64 environment, as the
current_cpu in the host toolchain is always x64.

Change-Id: Id2b21b3c983a78c4d29361dd1452400f9649e12b
diff --git a/build/testing/test_spec.gni b/build/testing/test_spec.gni
index 8581d55..c466f85 100644
--- a/build/testing/test_spec.gni
+++ b/build/testing/test_spec.gni
@@ -120,7 +120,7 @@
     platform_dims += [ empty_dim ]
     platform_dims -= [ empty_dim ]
 
-    if (!defined(platform.cpu) || platform.cpu == current_cpu) {
+    if (!defined(platform.cpu) || platform.cpu == target_cpu) {
       target_platform_dims += [ platform_dims ]
     } else {
       other_platform_dims += [ platform_dims ]