[sanitizer] Restrict querying VM size on Darwin only to iOS devices

We currently do this for SANITIZER_IOS, which includes devices *and* simulators. This change opts out the check for simulators to unify the behavior with macOS, because VM size is really a property of the host OS, and not the simulator.

<rdar://problem/72129387>

Differential Revision: https://reviews.llvm.org/D93140