MFC r328221:

Define a new __alloc_size2 attribute to complement the existing support.

At least on GCC7, calling __alloc_size(x) twice is not equivalent to
calling using the attribute once with two arguments. The later is the
documented use in GCC documentation so add a new alloc_size(n, x)
alternative to cover for the few places where it is used: basically:
calloc(3), reallocarray(3) and mallocarray(9).

Submitted by: Mark Millard