Fortran/OpenMP: Fix -fopenmp-simd for 'omp assume(s)'

While 'omp assume' is enabled by -fopenmp-simd, 'omp assumes' is not;
however, due to the way parsing works in Fortran (esp. for fixed-form
source code), 'assumes' was parsed by 'assume' which then stumbled over
the tailing 's'.

gcc/fortran/

	* parse.cc (decode_omp_directive): Really ignore 'assumes' with
	-fopenmp-simd.

gcc/testsuite/

	* gfortran.dg/gomp/openmp-simd-8.f90: New test.