Fix position of tests-unsupported definition in assert/Makefile.

tests-unsupported has to be defined before the inclusion of Rules in a
subdirectory Makefile; otherwise it is ineffective.  This patch fixes
the ordering in assert/Makefile, where a recent test addition put
tests-unsupported too late (resulting in build failures when the C++
compiler was missing or broken, and thereby showing up the unrelated
bug 21987).

Incidentally, I don't see why these tests depend on
$(have-cxx-thread_local) rather than just a working C++ compiler.

Tested in such a configuration (broken compiler/libstdc++) with
build-many-glibcs.py.

	* assert/Makefile [$(have-cxx-thread_local)]: Move conditional
	variable definitions above inclusion of ../Rules.

(cherry picked from commit 75dfe623)