libstdc++: only define _GLIBCXX_HAVE_TLS for VxWorks >= 6.6

libstdc++: only define _GLIBCXX_HAVE_TLS for VxWorks >= 6.6

According to
https://gcc.gnu.org/legacy-ml/gcc-patches/2008-03/msg01698.html, the
TLS support, including the __tls_lookup function, was added to VxWorks
in 6.6.

It certainly doesn't exist on our VxWorks 5 platform, but the fallback
code in eh_globals.cc using __gthread_key_create() etc. used to work
just fine.

libstdc++-v3/ChangeLog:

	* config/os/vxworks/os_defines.h (_GLIBCXX_HAVE_TLS): Only
	define for VxWorks >= 6.6.