gen_isr_tables: apply offset to irq parameter

The interrupts would be placed at incorrect offsets on systems where
some interrupt vectors are reserved for exceptions, such as ARC.

Change-Id: I5b1f00eb9e8aecb84ae66e3d0461a734ffb5fbe6
Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>