mask all interrupts when MASTER_DISABLE is set

mask all interrupts when MASTER_DISABLE is set

The MASTER_DISABLE bit (aka mask-all) masks all the interrupts.

According to Sun-4M System Architecture
"The level–15 interrupt sources [...] are maskable with the Interrupt Target
Mask Register. While these interrupts are considered ’non–maskable’ within
the SPARC IU, a mask capability is provided to allow the boot firmware
to establish a basic environment before receiving any level–15 interrupts,
which are non–maskable within SPARC. A mask–all bit is provided to allow
disabling of all external interrupts during change of the CIT."

Signed-off-by: Artyom Tarasenko <atar4qemu@gmail.com>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>