Add a separate GC stack for tracking GC preserved objects, and add hooks for GC preserve regions

[qinsoon]

This PR adds two things:

1. a different shadow stack to record objects that are GC preserved. We need to be able to identify those objects, and they are different from the objects in the normal stack.
2. GC-specific hooks that are inserted for GC.@preserve regions, and related refactoring for code gen. Each GC can deal with the semantics of GC.@preserve based on its implementation.

There could be an alternative implementation for 1. Instead of having a different shadow stack, we just use an extra bit to encode whether the objects is from GC preserve, or from normal GC push. This would require changing the stock GC to handle the extra bit. But this is definitely viable.