build: move GLOBAL_HOST_RUSTFLAGS to kernel build system am: 750734867c

Original change: https://android-review.googlesource.com/c/trusty/lk/trusty/+/2928630

Change-Id: Ic68cfedbc34a77835bcb1ec584e380e120d6d1e2
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>