clocksource/drivers/arm_arch_timer: Extract elf_hwcap use to arch-helper

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: Andrew Murray <andrew.murray@arm.com>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Acked-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>