This CL imports some gdbinit magic from Chromium's tools. This fixes
gdb warnings about differences between psymtab and symtab.

R=bmeurer@chromium.org
CC=leszeks@chromium.org

Change-Id: I06e67c17e03b803c516ab59aeb6c17435b81d6d0
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1611540Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Commit-Queue: Ben Titzer <titzer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#61492}