Add mutex feature for std variant am: 4e5cfa7cfc am: 9ee1c1c4ac

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

Change-Id: I7d10031109cdc34b9ea1c4a5e1d27ca57ee3c5fe
Signed-off-by: Automerger Merge Worker <[email protected]>