Port from tfs: onload exception debugger crash fix (dotnet/coreclr#4868)

Port from tfs: onload exception debugger crash fix (dotnet/coreclr#4868)

The ExceptionHijackPersonalityRoutine was retrieving a context from a
particular stack frame, but the calling conventions for the function
executing in that frame allow it to overwrite the context. This was
causing the debugger to crash for exceptions thrown from the onload
method in winforms on x64.

The fix is to instead retrieve the context from the previous stack
frame, whose layout is set up explicitly in ExceptionHijack.

Commit migrated from https://github.com/dotnet/coreclr/commit/f1d6f7abbc5dd716cea4dc75b3da8043b9759ded