Fallback to old behavior when marshaling a non-blittable fixed buffer (dotnet/coreclr...

Fallback to old behavior when marshaling a non-blittable fixed buffer (dotnet/coreclr#20575)

* Throw an exception only when attempting to marshal nonblittable fixed buffers to/from native instead of on all marshalling operations.

* Assert in debug and checked builds when trying to marshal a fixed buffer of a non-blittable type.

Commit migrated from https://github.com/dotnet/coreclr/commit/f7863cf3877a1ce17bde80b8e78ed5b9ba2cf8f5