Fix pthread mutex type from initializer on OSX

[sim642]

Fixes 71-doublelocking/19-default-init not passing in OSX CI.

OSX pthread_mutex_t doesn't have a .__data.__kind field where the mutex type is assigned on initialization. Rather there's a .__sig field with magic constants.
We already hard-code an OSX constant for PTHREAD_MUTEX_RECURSIVE, so such hard-coding is not new:

analyzer/src/cdomain/value/cdomains/mutexAttrDomain.ml

Line 25 in 50f5304

let res = ref (Z.of_int 2) in (* Use OS X as the default, it doesn't have the enum *)

A cleaner (but also weirder) solution would be to extract actual values configure-style (a la CIL's Machdep).