nptl_db: disable DT_RELR on libthread_db.so

Some nptl tests inadvertently use the host's gdb to verify
libthread_db.so, which is loaded with the host's runtime.  This causes
a couple of test failures when the host glibc does not support DT_RELR.

The not correct, but simple, workaround is to build without DT_RELR
as this library is otherwise likely to load on glibc 2.17 and newer
today.

This allows tst-pthread-gdb-attach{,-static} to continue working
when testing on a gdb loaded with an older glibc.

This avoids a failure in tst-pthread-gdb-attach similar to:

  Trying host libthread_db library: .../build/glibc/nptl_db/libthread_db.so.1.
  dlopen failed: /lib64/libc.so.6: version `GLIBC_ABI_DT_RELR' not found (required by .../build/glibc/nptl_db/libthread_db.so.1).

Reviewed-by: Carlos O'Donell <carlos@redhat.com>