* misc/sys/cdefs.h: Define __attribute_malloc__ according to
available gcc version.
* string/string.h: Mark strdup, __strdup, and strndup with
__attribute_malloc__.
* stdlib/stdlib.h: Make malloc, calloc, realloc, and valloc with
__attribute_malloc__.
* malloc/malloc.h: Make malloc, calloc, realloc, valloc, pvallc,
__morecore, and __default_morecore with __attribute_malloc__.
Provide default definition for __attribute_malloc__.
* libio/stdio.h: Make tempnam with __attribute_malloc__.