Linux: dirent/tst-readdir64-compat can be a regular test

compat_symbol_reference works in non-internal tests now.  Also
avoid building the test for unsupported configurations at all.
I verified by building with build-many-glibcs.py that GLIBC_2.1.3
works as the predecessor of GLIBC_2.2.  (Symbol versions in
the early days are complex.)

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