make: Disable libcxx kernel test am: f70b35d686

Original change: https://android-review.googlesource.com/c/trusty/lk/trusty/+/3453132

Change-Id: I9b51ac4d5df6ec2472f439fb3b726f44fbb9729e
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>