x86: don't set up early exception handlers for external interrupts

All of early setup runs with interrupts disabled, so there is no
need to set up early exception handlers for vectors >= 32

This saves some minor text size.

Signed-off-by: Andi Kleen <ak@suse.de>
Cc: mingo@elte.hu
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Ingo Molnar <mingo@elte.hu>