locking/atomics: Fix out-of-tree build

Building a kernel out of tree with:

  make O=/tmp/b oldconfig
  cd /tmp/b
  make

gives this error:

    CALL    /mnt/kernel/kernel/linux/scripts/atomic/check-atomics.sh
  /bin/bash: scripts/atomic/check-atomics.sh: No such file or directory
  make[3]: *** [/mnt/kernel/kernel/linux/./Kbuild:86: old-atomics] Error 127
  make[3]: *** Waiting for unfinished jobs....

Make the command use the proper build prerequisite which is the absolute
path to the script.

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Borislav Petkov <bp@suse.de>
Cc: Boqun Feng <boqun.feng@gmail.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Will Deacon <will.deacon@arm.com>
Cc: arnd@arndb.de
Cc: aryabinin@virtuozzo.com
Cc: catalin.marinas@arm.com
Cc: dvyukov@google.com
Cc: glider@google.com
Cc: linux-arm-kernel@lists.infradead.org
Cc: linuxdrivers@attotech.com
Fixes: 8d32588077bd ("locking/atomics: Check generated headers are up-to-date")
Link: http://lkml.kernel.org/r/20181108194128.13368-1-bp@alien8.de
Signed-off-by: Ingo Molnar <mingo@kernel.org>