[gn] Add pool for dsymutil

dsymutil seems to kill the machine when too many processes are run in
parallel, so we need to use a pool to limit the concurrency when
passing large -j to Ninja (e.g. Goma build).

Change-Id: I84404291db4781f86268256ab888cef8d07c770e
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/234477
Commit-Queue: Jim Van Verth <jvanverth@google.com>
Auto-Submit: Ben Wagner aka dogben <benjaminwagner@google.com>
Reviewed-by: Jim Van Verth <jvanverth@google.com>
3 files changed