calloc-gnu: Fix bug on 32-bit AIX (regression 2024-11-04). * lib/stdlib.in.h (calloc): Consider _GL_USE_STDLIB_ALLOC. * lib/calloc.c: Define _GL_USE_STDLIB_ALLOC. Don't undefine calloc. * lib/malloc.c: Add comment. * lib/realloc.c: Likewise.