Fortran: Fix OpenMP/OpenACC continue-line parsing

Fortran: Fix OpenMP/OpenACC continue-line parsing

gcc/fortran/ChangeLog:

	* scanner.c (skip_fixed_omp_sentinel): Set openacc_flag if
	this is not an (OpenMP) continuation line.
	(skip_fixed_oacc_sentinel): Likewise for openmp_flag and OpenACC.
	(gfc_next_char_literal): gfc_error_now to force error for mixed OMP/ACC
	continuation once per location and return '\n'.

gcc/testsuite/ChangeLog:

	* gfortran.dg/goacc/omp-fixed.f: Re-add test item changed in previous
	commit in addition - add more dg-errors and '... end ...' due to changed
	parsing.
	* gfortran.dg/goacc/omp.f95: Likewise.
	* gfortran.dg/goacc-gomp/mixed-1.f: New test.