Ensure that nothrow versions of new and delete call the ordinary
versions of new or delete, instead of calling malloc or free directly.

These files are all compiled with -std=gnu++14 so can use noexcept and
nullptr to make the code more readable.

	PR libstdc++/68210
	* doc/xml/manual/intro.xml: Document LWG 206 change.
	* libsupc++/del_op.cc: Replace _GLIBCXX_USE_NOEXCEPT with noexcept.
	* libsupc++/del_opa.cc: Likewise.
	* libsupc++/del_opant.cc: Likewise.
	* libsupc++/del_opnt.cc: Likewise. Call operator delete(ptr) instead
	of free(ptr).
	* libsupc++/del_ops.cc: Replace _GLIBCXX_USE_NOEXCEPT with noexcept.
	* libsupc++/del_opsa.cc: Likewise.
	* libsupc++/del_opva.cc: Likewise.
	* libsupc++/del_opvant.cc: Likewise.
	* libsupc++/del_opvnt.cc: Likewise. Call operator delete[](ptr)
	instead of operator delete(ptr).
	* libsupc++/del_opvs.cc: Replace _GLIBCXX_USE_NOEXCEPT with noexcept.
	* libsupc++/del_opvsa.cc: Likewise.
	* libsupc++/new_op.cc: Use __builtin_expect in check for zero size.
	* libsupc++/new_opa.cc: Use nullptr instead of literal 0.
	* libsupc++/new_opant.cc: Likewise. Replace _GLIBCXX_USE_NOEXCEPT
	with noexcept.
	* libsupc++/new_opnt.cc: Likewise. Call operator new(sz) instead of
	malloc(sz).
	* libsupc++/new_opvant.cc: Use nullptr and noexcept.
	* libsupc++/new_opvnt.cc: Likewise. Call operator new[](sz) instead of
	operator new(sz, nothrow).
	* testsuite/18_support/new_nothrow.cc: New test.

From-SVN: r263478