Do not use generic selection in C++ mode

The logic to protect the use of generic selection (_Generic) does not
check for C or C++ mode, however, generic selection is a C-only
feature.

Tested for powerpc64le.

	* misc/sys/cdefs.h (__HAVE_GENERIC_SELECTION): Define to 0, if
	in C++ mode.

(cherry picked from commit 6913ad65)