Create a stack for JS values and use it in the interpreter

Create a stack for JS values and use it in the interpreter

First step towards being able to do an exact GC.

Create a stack for JS Values that is separate from the C++
stack.

Use the stack for generated methods (masm and moth).

Change-Id: I80ac0e5b5d86439dda5e9ea2b21fa0c57d8aef22
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>