[asan][aarch64] Don't use 64 bit allocator for Apple ios family

The recent change (https://reviews.llvm.org/D137136) to unconditionally
choose the 64 bit allocator on aarch64 breaks Apple iOS family of devices
which purposely use a smaller address space than is used with macOS.

rdar://102527313

Reviewed By: thetruestblue, yln

Differential Revision: https://reviews.llvm.org/D139030