This adds a function GetHeterogeneousCpuInfo() to HAL, which
classifies the processor cores in to "big", "medium", and
"little". This is currently only implemented on Android, where it
works by parsing the maximum CPU frequency from sysfs.
When all CPUs have the same frequency they are all classified as
"big", and when there are only 2 different frequencies they are
classified as "big" and "little". All CPUs with a frequency in-between
the lowest and highest are classified as "medium".
This information can be used to count the number of each cores in each
category, eg for determining appropriate thread pool sizes. Or to
determine the indices of cores in a certain category, eg for setting
CPU affinity for certain threads.
Differential Revision: https://phabricator.services.mozilla.com/D188478
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