R2R ilstubs (#24823)

R2R ilstubs (#24823)

* Basic support for precompiled pinvoke stubs

* Generate R2R file with multiple references to same IL stub (one per method which the IL stub is associated with)
* Not all il stubs are p/invokes. Don't fail when they aren't.
* Consistently use IsDynamicScope and GetModule to avoid unsafe memory access in IL stub compilation paths
* Enable full p/invoke il stubs when compiling System.Private.Corelib
* Disable IL Stub generation in crossgen for ARM32.
- The cross bitness logic is not correct for IL Stub generation