posix/tst-rfc3484: Fix compile failure linking to local __stat64

After 04986243 ("Remove internal usage of extensible stat functions")
linking the __stat64 symbol in getaddrinfo for this test fails with the
below error:

    [...] or1k-smh-linux-gnu/bin/ld: [...]/posix/tst-rfc3484.o: in function `gaiconf_reload':
    [...]/sysdeps/posix/getaddrinfo.c:2136: undefined reference to `__stat64'
    collect2: error: ld returned 1 exit status

This is because __stat64 is a local symbol, the test includes the
getaddrinfo directly and fails to link against the local symbol.  Fix
this by setting up an alias to the global stat64 symbol name like is
done for other local symbol usage.

Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>