irq: threaded: added global irq disable.

The function calling irq_signal_threads only disables the local IRQ's
(sometimes there's no difference, tho). irq_signal_threads works on
global IRQ's, and thus, needs global IRQ's to be disabled. Fixed a
potential bug by added a global IRQ disable call to irq_signal_threads.

Signed-off-by: Michel Megens <dev@michelmegens.net>
ETA/OS 2.0.4

Signed-off-by: Michel Megens <dev@michelmegens.net>