irq: fix declared _sw_isr_table size on ARC

On ARC, the _sw_isr_table has an actual size of CONFIG_NUM_IRQS - 16
because the 16 first IRQ are reserved.

This patch has no impact on the generated binary but allows the
debugger to display valid information instead of displaying 16 false
entries containing garbage.

Change-Id: Iae2756c1bc333dc822e9f80c2115fba6521792a2
Signed-off-by: Fabien Chereau <fabien.chereau@intel.com>