bsp/leon3: Use printk() only if debug is enabled
bsp/leon3: Install IPI handler early

Install inter-processor interrupt (IPI) handler before secondary CPUs
are started.  This ensures that secondary CPUs fetch the newly installed
handler.

Remove superfluous return statement.
bsp/leon3: Fix SMP initialization

Avoid usage of the same stack area by multiple secondary processors at
the same time.

Avoid magic delay loops.
bsp/leon3: Avoid magic delays for IPI broadcast
bsps: Delete unused bsp_smp_delay()
score: _CPU_Context_switch_to_first_task_smp()

Delete _CPU_Context_switch_to_first_task_smp() and use
_CPU_Context_restore() instead.