Call __jit_debug_register_code() indirectly via global variable.

This is speculative future optimization. It allows the debugger to
change the value of the global variable to point to its injected code.
This, in turn, allows it to handle JIT events in-process without
triggering breakpoint and without the inter-process cost of it.

Change-Id: I4961c8862603bfb43c84974a3a51110ee202bb1b