Stop exposing the C11 _Atomic() macro in <sys/cdefs.h>, when compiling
for C++. It clashes with the one in libc++'s <atomic> header.

(Previously, the _Atomic() macro was defined in <stdatomic.h>, which is
only for use with C11, but for various reasons it was moved to its
current location in r251804.)

Discussed with: bdrewery, ed
MFC after: 2 weeks