green: Don't return the red zone in stack_bounds()

This is mostly just an implementation detail, and anyone worried about the stack
bounds doesn't need to be bothered with the red zone because it's not usable
anyway.

Closes #12897
auto merge of #12908 : alexcrichton/rust/issue-12897, r=thestinger

This is mostly just an implementation detail, and anyone worried about the stack
bounds doesn't need to be bothered with the red zone because it's not usable
anyway.

Closes #12897