malloc: Fix tst-tcfree3 build csky-linux-gnuabiv2 with fortify source

With gcc 13.1 with --enable-fortify-source=2, tst-tcfree3 fails to
build on csky-linux-gnuabiv2 with:

../string/bits/string_fortified.h: In function ‘do_test’:
../string/bits/string_fortified.h:26:8: error: inlining failed in call
to ‘always_inline’ ‘memcpy’: target specific option mismatch
   26 | __NTH (memcpy (void *__restrict __dest, const void *__restrict
      __src,
      |        ^~~~~~
../misc/sys/cdefs.h:81:62: note: in definition of macro ‘__NTH’
   81 | #  define __NTH(fct)    __attribute__ ((__nothrow__ __LEAF)) fct
      |                                                              ^~~
tst-tcfree3.c:45:3: note: called from here
   45 |   memcpy (c, a, 32);
      |   ^~~~~~~~~~~~~~~~~

Instead of relying on -O0 to avoid malloc/free to be optimized away,
disable the builtin.

Reviewed-by: DJ Delorie <dj@redhat.com>