Fix Interop/PInvoke/Miscellaneous/HandleRef tests under GCStress (dotnet/coreclr...

Fix Interop/PInvoke/Miscellaneous/HandleRef tests under GCStress (dotnet/coreclr#21131)

* Add GC.KeepAlive call in the IL stub when marshalling a HandleRef.

* Add a GC.KeepAlive call for the BoxedInt since it isn't kept alive when running GC Stress

* Reference HandleRef._handle via the binder instead of by field offset arithmetic

* Alignment

* Use Marshal.Alloc/FreeHGlobal instead of the weird no-pinning machinery that was failing before

* Update HandleRefTest.cs

Commit migrated from https://github.com/dotnet/coreclr/commit/d07633f53b51a30d294c4afe2b0b7ba5b563c134