malloc: Fix set-freeres.c with gcc 6

malloc: Fix set-freeres.c with gcc 6

Old GCC might trigger the the comparison will always evaluate as ‘true’
warnig for static build:

set-freeres.c:87:14: error: the comparison will always evaluate as
‘true’ for the address of ‘__libc_getgrgid_freemem_ptr’ will never be
NULL [-Werror=address]
   if (&__ptr != NULL)      \

So add pragma weak for all affected usages.

Checked on x86_64 and i686 with gcc 6 and 13.
Reviewed-by: Carlos O'Donell <carlos@redhat.com>