Fix GC reporting for slow tail call arguments of type `Span<T>` (#14826)

Fix GC reporting for slow tail call arguments of type `Span<T>` (#14826)

Fix GC reporting for slow tail call arguments of type `Span<T>`

Fixes https://github.com/dotnet/coreclr/issues/9032:
- Refactored by-ref-like method table walking to find offsets of by-ref pointers in siginfo.hpp/cpp
- Reused that for appending GC layout when creating the copy-args helper for a slow tail call