Using the "jco" gdb macro on a WasmCode address would currently crash.
With this change, you can also use it to print code objects off the gc
heap.

R=titzer@chromium.org
CC=wasm-team@google.com

Bug: v8:7140, v8:6876
Change-Id: I21daf58b6b4734e3d052b66284b95938b4f57d40
Reviewed-on: https://chromium-review.googlesource.com/796416Reviewed-by: Ben Titzer <titzer@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49753}