[tools] Set up source map for lldb

With relative paths in the debug symbols, lldb cannot find the source
files, so set up a source map to direct "../.." to V8 root. This is
similar to what Chromium does in src/tools/lldb/lldbinit.py.

Bug: v8:11879
Change-Id: Ic6126aacafa7e3462c69da538a9528041c92ef00
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2998517


Commit-Queue: Zhi An Ng <zhin@chromium.org>
Reviewed-by: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#75521}