arm64: add HWCAP_EVTSTRM and associated hwcap refactoring

Take the hwcaps changes from 46efe547aca8498d51b64460c02366ae4032ca32 to
facilitate cherry-picking later hwcaps changes, while skipping the timer
changes that actually enable event streams for now.  The timer changes
depend on some non-trivial changes made after 3.10, and can safely be
dropped: the kernel will just continue reporting that HWCAP_EVTSTRM is
not available.

Bug: 17431179

Change-Id: I41548846f8cd7ae8147a2b115cc0f84708e29552
Signed-off-by: Greg Hackmann <ghackmann@google.com>