[Mono] Implement support for CallConvSuppressGCTransition

[lambdageek]

See #38134. The CoreCLR implementation is in #46343

This gives the ability to declare a function pointer that can be called without a GC transition.

For example:

delegate* unmanaged[Cdecl, SuppressGCTransition]<int*, int> fptr
        = (delegate* unmanaged[Cdecl, SuppressGCTransition]<int*, int>)GetNextUIntFunctionPointer();

The call to GetNextUintFunctionPointer returns a pointer to some unmanaged function. By declaring its calling convention as [Cdecl, SuppressGCTransition] (as opposed to [Cdecl]) the caller code does not need to do a managed-to-native transition when calling through fptr. All the rules of SuppressGCTransitionAttribute still apply: the native function can't block, throw exceptions, call back into the runtime, manipulate locks or other concurrency primitives, etc.

---

Re-enable Interop/PInvoke/Attributes/SuppressGCTransition/SuppressGCTransitionTest/ in src/tests/issues.targets once this is fixed.

[lambdageek]

I'm not actually sure that mono today emits a GC transition for something like:

  int n;
  unsafe {
    delegate* unmanaged[Cdecl]<int*, int> fptr
        = (delegate* unmanaged[Cdecl]<int*, int>)GetNextUIntFunctionPointer();
    fptr(&n);
  }

We need to complete #38480 - the calli call for the last line in the example needs to emit a gc transition