Set omp_requires_mask for dynamic_allocators.

Set omp_requires_mask for dynamic_allocators.

This is backport of a patch posted in
https://gcc.gnu.org/pipermail/gcc-patches/2022-February/590655.html

This patch fixes an issue that although gfortran accepts
'requires dynamic_allocators', it does not set the omp_requires_mask
accordingly.

gcc/fortran/ChangeLog:

	* parse.cc (gfc_parse_file): Set OMP_REQUIRES_DYNAMIC_ALLOCATORS
	bit in omp_requires_mask.