Use the __alloc_size2 attribute where relevant.

This follows the documented use in GCC. It is basically only relevant for
calloc(3), reallocarray(3) and mallocarray(9).

Suggested by: Mark Millard

Reference:
https://docs.freebsd.org/cgi/mid.cgi?9DE674C6-EAA3-4E8A-906F-446E74D82FC4