arm64: add cpu_capabilities bitmap

For taking note if at least one CPU in the system needs a bug
workaround or would benefit from a code optimization, we create a new
bitmap to hold (artificial) feature bits.
Since elf_hwcap is part of the userland ABI, we keep it alone and
introduce a new data structure for that (along with some accessors).

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>

Cc: <stable@vger.kernel.org> # v3.18.y
(cherry picked from commit 930da09f5e50dd22fb0a8600388da8677d62d671)
Signed-off-by: Kevin Hilman <khilman@linaro.org>
Signed-off-by: Sasha Levin <sasha.levin@oracle.com>