C, C++, Fortran, OpenMP: Add support for 'flush seq_cst' construct.

C, C++, Fortran, OpenMP: Add support for 'flush seq_cst' construct.

This patch adds support for the 'seq_cst' memory order clause on the 'flush'
directive which was introduced in OpenMP 5.1.

gcc/c-family/ChangeLog:

	* c-omp.c (c_finish_omp_flush): Handle MEMMODEL_SEQ_CST.

gcc/c/ChangeLog:

	* c-parser.c (c_parser_omp_flush): Parse 'seq_cst' clause on 'flush'
	directive.

gcc/cp/ChangeLog:

	* parser.c (cp_parser_omp_flush): Parse 'seq_cst' clause on 'flush'
	directive.
	* semantics.c (finish_omp_flush): Handle MEMMODEL_SEQ_CST.

gcc/fortran/ChangeLog:

	* openmp.c (gfc_match_omp_flush): Parse 'seq_cst' clause on 'flush'
	directive.
	* trans-openmp.c (gfc_trans_omp_flush): Handle OMP_MEMORDER_SEQ_CST.

gcc/testsuite/ChangeLog:

	* c-c++-common/gomp/flush-1.c: Add test case for 'seq_cst'.
	* c-c++-common/gomp/flush-2.c: Add test case for 'seq_cst'.
	* g++.dg/gomp/attrs-1.C: Adapt test to handle all flush clauses.
	* g++.dg/gomp/attrs-2.C: Adapt test to handle all flush clauses.
	* gfortran.dg/gomp/flush-1.f90: Add test case for 'seq_cst'.
	* gfortran.dg/gomp/flush-2.f90: Add test case for 'seq_cst'.