Linux: dirent/tst-readdir64-compat needs to use TEST_COMPAT (bug 27654)

The hppa port starts libc at GLIBC_2.2, but has earlier symbol
versions in other shared objects.  This means that the compat
symbol for readdir64 is not actually present in libc even though
have-GLIBC_2.1.3 is defined as yes at the make level.

Fixes commit 15e50e6c ("Linux:
dirent/tst-readdir64-compat can be a regular test") by mostly
reverting it.