[AArch64][compiler-rt] Avoid use of libc header in sme-libc-routines (#79454)

The use of `#include <stdlib.h>` introduces a libc dependency. In many
build environments such a file can be found under e.g. /usr/include, but
this does not necessarily correspond to the libc in use, which may not
be available until after the builtins have been built.

So far as I understand, it's not valid to have a dependency on libc from
builtins; there are a handful of such includes in builtins, but they are
protected by ifdefs.

Instead, use <stddef.h>, which provides `size_t` and is provided by the
compiler's resource headers and so should always be available.