MN10300: Provide the functions to fully disable maskable interrupts

The local_irq_disable() function and co. merely raise the interrupt mask on the
MN10300 arch to exclude normal interrupts.  This still lets other, higher
priority maskable interrupts through, such as are used to service gdbstub's
serial port and the MN10300 on-chip serial port virtual FIFOs.

Provide functions to allow the maskable interrupts to be fully disabled, which
will exclude those interrupts.

Signed-off-by: Akira Takeuchi <takeuchi.akr@jp.panasonic.com>
Signed-off-by: Kiyoshi Owada <owada.kiyoshi@jp.panasonic.com>
Signed-off-by: David Howells <dhowells@redhat.com>