The aligned allocation and deallocation functions were added in the dylib
in Mac OSX 10.13. This commit does two things: first, it adds availability
markup to those functions such that clients targetting older OSes get a
compiler error instead of a link error if they try to use these functions.
Secondly, it makes sure that the library itself does not use these functions
when targetting older OSes.
rdar://problem/34223934
rdar://problem/42826941