Fortran/OpenMP: Support 'omp parallel master'

gcc/fortran/ChangeLog:

	* dump-parse-tree.c (show_omp_node, show_code_node): Handle
	EXEC_OMP_PARALLEL_MASTER.
	* frontend-passes.c (gfc_code_walker): Likewise.
	* gfortran.h (enum gfc_statement): Add ST_OMP_PARALLEL_MASTER and
	ST_OMP_END_PARALLEL_MASTER.
	(enum gfc_exec_op): Add EXEC_OMP_PARALLEL_MASTER..
	* match.h (gfc_match_omp_parallel_master): Handle it.
	* openmp.c (gfc_match_omp_parallel_master, resolve_omp_clauses,
	omp_code_to_statement, gfc_resolve_omp_directive): Likewise.
	* parse.c (decode_omp_directive, case_exec_markers,
	gfc_ascii_statement, parse_omp_structured_block,
	parse_executable): Likewise.
	* resolve.c (gfc_resolve_blocks, gfc_resolve_code): Likewise.
	* st.c (gfc_free_statement): Likewise.
	* trans-openmp.c (gfc_trans_omp_parallel_master,
	gfc_trans_omp_workshare, gfc_trans_omp_directive): Likewise.
	* trans.c (trans_code): Likewise.

libgomp/ChangeLog:

	* testsuite/libgomp.fortran/parallel-master.f90: New test.

gcc/testsuite/ChangeLog:

	* gfortran.dg/gomp/parallel-master-1.f90: New test.
	* gfortran.dg/gomp/parallel-master-2.f90: New test.