autoninja automatically sets -l <num_cores>.  -l option makes ninja
not invoke a new command if current CPU load average is above -l.

However, as far as I investigated, -l <num_cores> make the build
much slower than without -l especially on Linux & OSX machines with
small numbers of cores.  I should say -l decreases the build
performance.  When I build Chromium with Goma with the same -j with
autoninja without -l, the load average goes more than the number of
cores while keeping the machine working as usual.
Also, ninja can invoke commands until the spike of command invocation
is reflected to load average, -l might not mitigate for a machine
to get stuck by too high load.

Note that from what I understand from the implementation, Windows
ninja's posix-compatible load average is always less than num_cores.
i.e. -l <num_cores> won't limit the process invocation.
https://github.com/ninja-build/ninja/blob/59849864592b421e0a8f993011e7e5c2ab27e77b/src/util.cc#L479

Let me make autoninja not set -l and bring better build performance
on machines with small number of cores.

Bug: b/117810340
Change-Id: I50f231f1a8976f8ecfc3a0c778f0f1ac98d3827f
Reviewed-on: https://chromium-review.googlesource.com/c/1290611Reviewed-by: Andrii Shyshkalov <tandrii@chromium.org>
Reviewed-by: Bruce Dawson <brucedawson@chromium.org>
Commit-Queue: Yoshisato Yanagisawa <yyanagisawa@chromium.org>