Add a separate mutex for the {debug_side_tables_} field. This ensures
that we can use {GetDebugSideTableIfExists} even if {mutex_} is already
locked.

R=ahaas@chromium.org
CC=​​clemensb@chromium.org

Bug: v8:10889
Cq-Include-Trybots: luci.v8.try:v8_linux64_tsan_rel_ng
Cq-Include-Trybots: luci.v8.try:v8_linux64_tsan_isolates_rel_ng
Change-Id: Icb67c45aec0cf66814705b83532f4833f36738e1
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2402879
Commit-Queue: Thibaud Michaud <thibaudm@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#69888}