Span: VM side of GC reporting for `Span<T>` - investigate struct passing in regs on Unix

[kouvel]

Investigate if there is an issue with structs being passed in registers on Unix:

• Passing a span as an argument in registers (potentially fix ReportPointersFromStructInRegisters to cover ByRefLike types)
• Returning a span in registers during GC suspension

[kouvel]

Looks like returning in registers during GC suspension is not an issue