Instead of querying the platform for the number of available threads,
and allocating exactly N+1 queues, do grow the number of queues
dynamically. This allows for more than N+1 concurrent threads,
which then allows us to contribute to compilation instead of waiting
doing nothing. This will be added in a follow-up CL.

Special care is being taken to not synchronize too much between threads.
We take a shared mutex whenever stealing tasks, but not on the default
path where we pick a unit from the task's own queue.

R=thibaudm@chromium.org
CC=​etiennep@chromium.org

Bug: v8:11005
Change-Id: I1f67f15fb22b95ef246c37eb80c03132d8a1d149
Cq-Include-Trybots: luci.v8.try:v8_linux64_tsan_rel_ng
Cq-Include-Trybots: luci.v8.try:v8_linux64_tsan_isolates_rel_ng
Cq-Include-Trybots: luci.v8.try:v8_linux_gc_stress_dbg_ng
Cq-Include-Trybots: luci.v8.try:v8_mac64_gc_stress_dbg_ng
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2467844
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Reviewed-by: Thibaud Michaud <thibaudm@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70508}