Fix math/Makefile dependency on libm-test.stmp for libmvec tests.

Fix math/Makefile dependency on libm-test.stmp for libmvec tests.

Since the libmvec tests went in I've noticed build failures from
parallel testing in math/, when those tests start building before
libm-test.c has been fully generated.  (This only applies if libm test
sources have been modified after the original glibc build, because
otherwise libm-test.stmp was generated during the original build and
doesn't get regenerated during testing.)

Those tests depend on libm-test.stmp, but the dependency uses
$(libmvec-tests), which is set in the sysdeps Makefile fragments, and
appears before the inclusion of ../Rules, which is what includes those
fragments; thus, the dependency does not work and parallel make can
start building the vector tests too soon.  This patch moves the
dependency further down so that the required variable is defined when
the dependency is.

Tested for x86_64.

	* math/Makefile [$(PERL) != no]
	($(addprefix $(objpfx), $(addsuffix .o, $(libm-vec-tests)))): Move
	dependency on libm-test.stmp below the inclusion of Rules.