Support more than 64 CPU features.

After recent patches, we have exactly 64 CPU features. This patch makes
the mechanism flexible so that we can support more features in the
future.

Several operators on CPUFeatures had to be re-written as part of this,
so this patch replaces the default-argument implementations with a more
flexible template-based approach, which can accept more than four
features. Existing usage remains unaffected.

Change-Id: If91a3adb62669aa827464e857a90eb93a64db7a6