Two usability improvements to the GDB jco macro:

 * Check if the desired pc is within the code space (or large object
   space), to avoid failures
 * Highlight the current pc in the outputted code (yellow and bold) to
   make it easier to find.

Change-Id: Ia094f33b61ed0fd2dd1e5e456992a17d97048639
Reviewed-on: https://chromium-review.googlesource.com/860102
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50514}