ARM: 8455/1: define __BUG as asm(BUG_INSTR) without CONFIG_BUG

Following (a long time after) a4b5d580

 ("bug: Make BUG() always stop
the machine"), this adapts the ARM architecture to no longer rely
on the sub-optimal BUG() definition that has a silent endless loop
but instead use the same trapping instruction that we have for
the full BUG() support.

This avoids hundreds of warnings like

arch/arm/include/asm/xen/page.h: In function 'arbitrary_virt_to_machine':
arch/arm/include/asm/xen/page.h:85:1: warning: no return statement in function returning non-void [-Wreturn-type]

and also makes the code size slightly smaller. The behavior changes
from silently stopping the kernel to an oops, and follows what x86
does these days.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>