ARM: smp_twd: make local_timer_stop a symbol instead of a #define

When CONFIG_HAVE_ARM_TWD is selected, local_timer_stop is a #define,
while all other local timers are using a real function.

Convert it to an alias of twd_timer_stop, as it helps converting
all local timers to another internal API in a sane way.

Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>