Fix tls build in linuxthreads_db

[Roland McGrath]

> Not supported?  Then we should disable it.  Why it is not supported?

--with-tls is not the default.  It's not supported because it's not all
finished, the tools aren't all finished, etc.

> In that case I guess this failure running make check is known:
> 
> arthur:/cvs/libc/dlfcn:[1]$ LD_DEBUG=all GCONV_PATH=/builds/glibc/main-with-tls/iconvdata LC_ALL=C LD_LIBRARY_PATH=/builds/glibc/main-with-tls/dlfcn/:/builds/glibc/main-with-tls/:/builds/glibc/main-with-tls/elf  /builds/glibc/main-with-tls/dlfcn/tststatic 
> libc.so.6: shared object cannot be dlopen()ed

Yes.  That test will probably never work with TLS.