test/timer: use compiler atomic builtins for sync

test/timer: use compiler atomic builtins for sync

Convert rte_atomic usages to compiler atomic
built-ins for lcore_state and collisions sync.

Also, move 'main_init_workers' outside of
'timer_stress2_main_loop' to guarantee lcore_state
initialized correctly before the threads launched.

Signed-off-by: Joyce Kong <joyce.kong@arm.com>
Reviewed-by: Ruifeng Wang <ruifeng.wang@arm.com>
Reviewed-by: Honnappa Nagarahalli <honnappa.nagarahalli@arm.com>