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Normally, we allow running several links in parallel if there's enough
memory, since links tend to be memory- and io-bound, not compute bound.
However, in LTO builds the linker does codegen for every linked
translation unit, which needs a lot of CPU.

Previously, we used to hardcode 8 threads per link for thinlto builds,
which makes builds needlessly slow on machines with many cores.

Instead, automatically use all cores for a thinlto link, but only run
one link in parallel at once.

chromeos-chrome-9999.ebuild in the Chrome OS build and link_jobs_32 in
mb_config.pyl currently manually set the number of threads per lto link
to the number of cores. Once this change here is in, we should stop
explicitly setting the arg there and then we can turn this from a gn arg
to a regular variable that's not settable in args.gn

Bug: 1132930
Change-Id: If2fda3e38b2b1827beb2da844f024f52211210db
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2464782Reviewed-by: Hans Wennborg <hans@chromium.org>
Commit-Queue: Nico Weber <thakis@chromium.org>
Auto-Submit: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#816129}