OpenMP/Fortran: Extend reject code between target + teams [PR71065, PR110725]

The previous version failed to diagnose when the 'teams' was nested
more deeply inside the target region, e.g. inside a DO or some
block or structured block.

            PR fortran/110725
            PR middle-end/71065

gcc/fortran/ChangeLog:

	* openmp.cc (resolve_omp_target): Minor cleanup.
	* parse.cc (decode_omp_directive): Find TARGET statement
	also higher in the stack.

gcc/testsuite/ChangeLog:

	* gfortran.dg/gomp/teams-6.f90: Extend.