OpenMP/Fortran: Permit impure ELEMENTAL in omp directives

OpenMP/Fortran: Permit impure ELEMENTAL in omp directives

OpenMP since 4.5 permits IMPURE ELEMENTAL in directives and
the code already only checked for PURE. – Followup for
-fopenmp-simd.

gcc/fortran/ChangeLog:

	* parse.c (decode_omp_directive): Remove "or ELEMENTAL"
	from "in PURE" error message also for -fopenmp-simd.

gcc/testsuite/ChangeLog:

	* gfortran.dg/gomp/pr79154-simd.f90: New test.