Fix call to [get|record]_sp_limit for thumb arch

[bharrisau]

Now with #12407 thumb is supported for rust_stack_exhausted(). This calls get_sp_limit() which calls the implementation in rt/arch/arm/record_sp.S. This code isn't supported for thumb targets though. The call should be returning *STACK_LIMIT for thumb.

[alexcrichton]

We've since removed these functions from the runtime, so closing.