GitLab

https://chromium-review.googlesource.com/c/chromium/src/+/2464913 made
it so that thinlto links can use all available cores, and reduced the
number of parallel LTO links to 1.

This made individual links 30% to 60% faster, but the reduced parallelism
caused bots to still got much slower in aggregate.

Based on some handwaving, only a third of a thinlto link seems to be
CPU-bound (https://bugs.chromium.org/p/chromium/issues/detail?id=1132930#c14),
so try running up to 3 thinlto links in parallel.

I expect we'll have to iterate on this heuristic some more.

(And we might want to limit ourselves to 1 parallel thinlto link on
Windows eventually, see first bug below.)

Bug: 1132930,1137812
Change-Id: I77d9446195be4fb1171b8beba45205e91c89a875
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2467360
Commit-Queue: Nico Weber <thakis@chromium.org>
Auto-Submit: Nico Weber <thakis@chromium.org>
Reviewed-by: Hans Wennborg <hans@chromium.org>
Cr-Commit-Position: refs/heads/master@{#816565}