Fix `Span<T>` GCStress issue on x86 (dotnet/coreclr#9116)

Fix `Span<T>` GCStress issue on x86 (dotnet/coreclr#9116)

x86 by default treats the type of `ByReference<T>` as the actual type of its IntPtr field as an optimization. `ByReference<T>` is a special case where the struct needs to be treated as a value type so that its field can be considered as a by-ref pointer. This fixes `GCStress=f` issues on span tests on x86.

Commit migrated from https://github.com/dotnet/coreclr/commit/6f7baa9e1ec9a848c1b98a4af8d9d8c2851ef023