Add mutex feature for std variant am: 4e5cfa7cfc am: 207a35def4 am: 7213272174

Original change: https://android-review.googlesource.com/c/platform/external/rust/crates/spin/+/2842870

Change-Id: Ib3aef05eca7a5c33572e10454e3f82ac58538eab
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>