When replaying, for each batch a limited number of compile processes
is launched, but no all-over limit is set, so that on computers with
a low processor count and/or limited working memory the machine may
become un-responsive. Therefore, tell ninja to not spawn new processes
when the load is larger than the number of available processors.

Bug: angleproject:6078
Change-Id: I9982a139ef2346d0cf25fbd510c7aff038e48071
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2976658Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Cody Northrop <cnorthrop@google.com>
Commit-Queue: Gert Wollny <gert.wollny@collabora.com>