Both tables are always updated together and are always accessed
together. Thus merge them, reducing code complexity, but also code
space and overhead for accessing them during runtime. Instead of two
weak global handles, we only need one, which also means one less load
for each indirect call.
Merging them also improves cache locality, since signature and code
address are not stored next to each other in memory, so they will very
likely end up in the same cache line.

R=titzer@chromium.org

Change-Id: I862df7de93a98aa602a3895796610c2c520d6f21
Reviewed-on: https://chromium-review.googlesource.com/866868
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Ben Titzer <titzer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50623}