Fortran/OpenMP: Support most of 5.1 atomic extensions

Implements moste of OpenMP 5.1 atomic extensions,
except that 'compare' is parsed but rejected during
resolution. (As the trans-openmp.c handling is missing.)

gcc/fortran/ChangeLog:

	* dump-parse-tree.c (show_omp_clauses): Handle
	weak/compare/fail clause.
	* gfortran.h (gfc_omp_clauses): Add weak, compare, fail.
	* openmp.c (enum omp_mask1, gfc_match_omp_clauses,
	OMP_ATOMIC_CLAUSES): Update for new clauses.
	(gfc_match_omp_atomic): Update for 5.1 atomic changes.
	(is_conversion): Support widening in one go.
	(is_scalar_intrinsic_expr): New.
	(resolve_omp_atomic): Update for 5.1 atomic changes.
	* parse.c (parse_omp_oacc_atomic): Update for compare.
	* resolve.c (gfc_resolve_blocks): Update asserts.
	* trans-openmp.c (gfc_trans_omp_atomic): Handle new clauses.

gcc/testsuite/ChangeLog:

	* gfortran.dg/gomp/atomic-2.f90: Move now supported code to ...
	* gfortran.dg/gomp/atomic.f90: here.
	* gfortran.dg/gomp/atomic-10.f90: New test.
	* gfortran.dg/gomp/atomic-12.f90: New test.
	* gfortran.dg/gomp/atomic-15.f90: New test.
	* gfortran.dg/gomp/atomic-16.f90: New test.
	* gfortran.dg/gomp/atomic-17.f90: New test.
	* gfortran.dg/gomp/atomic-18.f90: New test.
	* gfortran.dg/gomp/atomic-19.f90: New test.
	* gfortran.dg/gomp/atomic-20.f90: New test.
	* gfortran.dg/gomp/atomic-22.f90: New test.
	* gfortran.dg/gomp/atomic-24.f90: New test.
	* gfortran.dg/gomp/atomic-25.f90: New test.
	* gfortran.dg/gomp/atomic-26.f90: New test.

libgomp/ChangeLog

	* libgomp.texi (OpenMP 5.1): Update status.