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.

Change-Id: I562c7115118c0587d594d4d5b62d25101e47bfd8
1 file changed