Different mechanisms are used to test and set elf_hwcaps between ARM
and ARM64, this results in the use of ifdeferry in this file when
setting/testing for the EVTSTRM hwcap.

Let's improve readability by extracting this to an arch helper.
Signed-off-by: NAndrew Murray <andrew.murray@arm.com>
Acked-by: NMark Rutland <mark.rutland@arm.com>
Acked-by: NWill Deacon <will.deacon@arm.com>
Signed-off-by: NDaniel Lezcano <daniel.lezcano@linaro.org>