microblaze: Support brki rX, 0x18 for user application debugging

This is the first patch which add support for
user application debugging through brki rX, 0x18 vector.

This patch has side effect which also remove security issue
to use brki rX, 0x18 to freeze kernel.

Support for old gdb support via priviledged exception
(brk r0, r0) is still there. It will be remove in future.

Signed-off-by: Michal Simek <monstr@monstr.eu>