<sys/cdefs.h>: Add support for alloc_align, alloc_size and malloc attributes.

<sys/cdefs.h>: Add support for alloc_align, alloc_size and malloc attributes.

  * __alloc_align tells the compiler that one of the arguments is a
    minimal alignment for pointer to help with pointers analysis.
  * __alloc_size tells the compiler that one of the arguments is a
    size of a pointer returned by function.
  * __malloclike tells the compiler that returned pointer does not
    alias any other pointer to help futher optimizations.

While there, add partial support for clang (as it is in FreeBSD).