x86: Lock interrupts when connecting interrupts

When dyanmicaly connecting an interrupt via irq_connect(), interrupts must be
locked both when allocating a dynamic interrupt stub and when updating the
interrupt descriptor table (IDT) to prevent race conditions.

Change-Id: I9c68c1f73dda478880f1a32793a84cb6eb87735e
Signed-off-by: Peter Mitsis <peter.mitsis@windriver.com>