Add mutex feature for std variant am: 4e5cfa7cfc am: 9ee1c1c4ac am: ac91fde56d

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

Change-Id: I16f72975a4a7380bab6e7e40cfd87f4f43f8e93a
Signed-off-by: Automerger Merge Worker <[email protected]>