[mtx_t][threadsafety] Add thread safety annotations to mtx_t

This adds locking annotations to mtx_lock and mtx_unlock. This means
that functions which return holding more locks than they started with
(or with fewer) must get annotations. A few cases in which locks are
conditionally acquired is beyond the analysis, and marked as such.

This does _not_ mean that any data is marked as guarded by a mtx_t,
but it does enable that.

Change-Id: Ia545f1515e238ab690888f98b84b7392ba809c4f

[musl] Move internal use of mxr_mutex_t to C11's mtx_t

Change-Id: Ie0fec1635c6b893f1d484d098a78c4d2de4990c3

[magenta] Initial commit