locking/atomics: Switch to generated fallbacks

As a step to ensuring the atomic* APIs are consistent, switch to fallbacks
generated by gen-atomic-fallback.sh.

These are checked in rather than generated with Kbuild, since:

* This allows inspection of the atomics with git grep and ctags on a
  pristine tree, which Linus strongly prefers being able to do.

* The fallbacks are not affected by machine details or configuration
  options, so it is not necessary to regenerate them to take these into
  account.

* These are included by files required *very* early in the build process
  (e.g. for generating bounds.h), and we'd rather not complicate the
  top-level Kbuild file with dependencies.

The new fallback header should be equivalent to the old fallbacks in
<linux/atomic.h>, but:

* It is formatted a little differently due to scripting ensuring things
  are more regular than they used to be.

* Fallbacks are now expanded in-place as static inline functions rather
  than macros.

* The prototypes for fallbacks are arragned consistently with the return
  type on a separate line to try to keep to a sensible line length.

There should be no functional change as a result of this patch.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Will Deacon <will.deacon@arm.com>
Cc: linux-arm-kernel@lists.infradead.org
Cc: catalin.marinas@arm.com
Cc: linuxdrivers@attotech.com
Cc: dvyukov@google.com
Cc: Boqun Feng <boqun.feng@gmail.com>
Cc: arnd@arndb.de
Cc: aryabinin@virtuozzo.com
Cc: glider@google.com
Link: http://lkml.kernel.org/r/20180904104830.2975-3-mark.rutland@arm.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>