Fortran: Update omp atomic for OpenMP 5

Fortran: Update omp atomic for OpenMP 5

gcc/fortran/ChangeLog:

	* dump-parse-tree.c (show_omp_clauses): Handle atomic clauses.
	(show_omp_node): Call it for atomic.
	* gfortran.h (enum gfc_omp_atomic_op): Add GFC_OMP_ATOMIC_UNSET,
	remove GFC_OMP_ATOMIC_SEQ_CST and GFC_OMP_ATOMIC_ACQ_REL.
	(enum gfc_omp_memorder): Replace OMP_MEMORDER_LAST by
	OMP_MEMORDER_UNSET, add OMP_MEMORDER_SEQ_CST/OMP_MEMORDER_RELAXED.
	(gfc_omp_clauses): Add capture and atomic_op.
	(gfc_code): remove omp_atomic.
	* openmp.c (enum omp_mask1): Add atomic, capture, memorder clauses.
	(gfc_match_omp_clauses): Match them.
	(OMP_ATOMIC_CLAUSES): Add.
	(gfc_match_omp_flush): Update for 'last' to 'unset' change.
	(gfc_match_omp_oacc_atomic): Removed and placed content ..
	(gfc_match_omp_atomic): ... here. Update for OpenMP 5 clauses.
	(gfc_match_oacc_atomic): Match directly here.
	(resolve_omp_atomic, gfc_resolve_omp_directive): Update.
	* parse.c (parse_omp_oacc_atomic): Update for struct gfc_code changes.
	* resolve.c (gfc_resolve_blocks): Update assert.
	* st.c (gfc_free_statement): Also call for EXEC_O{ACC,MP}_ATOMIC.
	* trans-openmp.c (gfc_trans_omp_atomic): Update.
	(gfc_trans_omp_flush): Update for 'last' to 'unset' change.

gcc/testsuite/ChangeLog:

	* gfortran.dg/gomp/atomic-2.f90: New test.
	* gfortran.dg/gomp/atomic.f90: New test.