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

sys/sys/cdefs.h

@@ -275,7 +275,8 @@
 #define	_Alignof(x)		__alignof(x)
 #endif
 
-#if !__has_extension(c_atomic) && !__has_extension(cxx_atomic)
+#if !defined(__cplusplus) && !__has_extension(c_atomic) && \
+    !__has_extension(cxx_atomic)
 /*
  * No native support for _Atomic(). Place object in structure to prevent
  * most forms of direct non-atomic access.