alpha: use generic current.h

Given that the arch does not add its own implementations, simply use the
asm-generic/current.h (generic-y) header instead of duplicating code.

Link: http://lkml.kernel.org/r/1485992878-4780-2-git-send-email-dave@stgolabs.net


Signed-off-by: Davidlohr Bueso <dbueso@suse.de>
Cc: Richard Henderson <rth@twiddle.net>
Cc: Ivan Kokshaysky <ink@jurassic.park.msu.ru>
Cc: Matt Turner <mattst88@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>