Undefine _Atomic before redefining

Stdatomic.h was potentially redefining _Atomic, in spite of a
prior definition by <atomic>.  This could cause g++ builds that
included <stdatomic.h> with an available <atomic> header to break.

A functional stdatomic.h is a prerequisite for fixing the
following bugs.  This is the middle of 3 AOSP updates to
bionics stdatomic.h that are needded to get there.

Bug:16880454
Bug:16513433

Change-Id: I562c7115118c0587d594d4d5b62d25101e47bfd8
(cherry picked from commit 3e4a0099)