Add CPUFeatures up to Armv8.6.

This adds support for all relevant features described in the latest
Armv8.6 XML.

Note that this removes the CPUFeatures::All() part of the
`API_CPUFeatures_format` test. It added little value to the test, and
was a burden to update when new features are added.

Change-Id: I276a0970be94c3adf2d0100874df0b82c7424a9b