Define OSATOMIC_USE_INLINED to get usable atomics on DARWIN

Define OSATOMIC_USE_INLINED to get usable atomics on DARWIN

In Mac OS 10.12, legacy interfaces for atomic operations have been
deprecated. Defining OSATOMIC_USE_INLINED gets us inline implementations
of the OSAtomic interfaces in terms of the <stdatomic.h> primitives.
This is a transition convenience.

Also indent preprocessor directives within the main DARWIN block to
improve readability.

Change-Id: Id10ae007d5427486f1b0a307a04a90f263201150
Reviewed-on: https://gerrit.openafs.org/12433
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>