alpha: Convert BUG() to use unreachable()

Use the new unreachable() macro instead of for(;;);

Signed-off-by: David Daney <ddaney@caviumnetworks.com>
CC: Richard Henderson <rth@twiddle.net>
CC: Ivan Kokshaysky <ink@jurassic.park.msu.ru>
CC: linux-alpha@vger.kernel.org
Signed-off-by: Matt Turner <mattst88@gmail.com>