drivers/timer/xtensa_sys_timer: Add hook for old-style interrupt handling

We still have one platform using (for now) the pre-asm2 integration
where the timer interrupt was handled via custom assembly.  It calls a
function named "_timer_int_handler" always, not the one we register
with IRQ_CONNECT.

Signed-off-by: Andy Ross <andrew.j.ross@intel.com>