Add C++-conforming wrappers for stdlib.h and math.h

Add C++-conforming wrappers for stdlib.h and math.h

	PR libstdc++/14608
	PR libstdc++/60401
	* include/Makefile.am: Use c_compatibility math.h and stdlib.h for
	--enable-cheaders=c_global configs.
	* include/Makefile.in: Regenerate.
	* include/c_compatibility/math.h: Remove obsolete _GLIBCXX_NAMESPACE_C
	test and allow inclusion from C files.
	* include/c_compatibility/stdlib.h: Likewise. Support freestanding.
	(at_quick_exit, quick_exit): Add using directives.
	* include/c_global/cmath: Use #include_next for math.h.
	* include/c_global/cstdlib: Use #include_next for stdlib.h.
	* testsuite/26_numerics/headers/cmath/14608.cc: New.
	* testsuite/26_numerics/headers/cmath/c99_classification_macros_c.cc:
	Remove xfail for most targets.
	* testsuite/26_numerics/headers/cstdlib/60401.cc: New.

From-SVN: r232586