Fixes problems on systems with opaque mutex

On some systems (iOS for example) mutex is represented by
opaque data structure which doesn't play well with simple
data copy. Therefore mutex should be initialized from
magic static value and filled by OS only when it landed RC.
auto merge of #14396 : vhbit/rust/opaque-mutex, r=alexcrichton

On some systems (iOS for example) mutex is represented by opaque data structure which doesn't play well with simple data copy. Therefore mutex should be initialized from magic static value and filled by OS only when it landed RC.

Initially written for iOS but since landing iOS support might require quite a lot of time I think it is better to split parts which aren't directly related to iOS and merge them in