OpenMP: allow requires dynamic_allocators

OpenMP: allow requires dynamic_allocators

There's no need to reject the dynamic_allocators requires directive because
we actually do support the feature, and it doesn't have to actually "do"
anything.

gcc/c/ChangeLog:

	* c-parser.c (c_parser_omp_requires): Don't "sorry" dynamic_allocators.

gcc/cp/ChangeLog:

	* parser.c (cp_parser_omp_requires): Don't "sorry" dynamic_allocators.

gcc/fortran/ChangeLog:

	* openmp.c (gfc_match_omp_requires): Don't "sorry" dynamic_allocators.

gcc/testsuite/ChangeLog:

	* gfortran.dg/gomp/requires-8.f90: Reinstate dynamic allocators
	requirement.