Implemented some basic caching to avoid unnecessary AOT compilation
of unchanged assemblies that were already compiled previously.
This reduces iOS export times considerably for subsequent builds
since many dependencies never change, such as framework assemblies
and the Godot bindings.
The AOT compiler asm output and object files are now placed in
`res://.mono/temp/obj/<CONFIG>/godot-aot-cache/` instead of a
temporary directory.
e72fdc4b95