Android: implement support for pthread synchronization primitives

[RalfJung]

We don't currently support the pthread shims on Android. This shouldn't be too hard: the shims are implemented in src/shims/unix/sync.rs, and all that needs to be done is figure out how big the pthread types are on Android and then picking suitable offsets for Miri to store its data in there.

The test command for this is ./miri --target aarch64-linux-android pthread-sync.

[RalfJung]

FWIW for 32bit android this is more tricky since pthread_mutex_t is too small for the hacks we currently use -- that would need #3749 before we can have proper support.

But so far we anyway only test 64bit Android on CI.