lib: extmem: rust: Move from external/lk/lib/rust_support/extmem.rs am: 91b7ad8fbc

Original change: https://android-review.googlesource.com/c/trusty/lk/trusty/+/3504162

Change-Id: Ie4eee06426d321a1c6faba9610720ea268e99738
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>