Fortran: Fix parse-dump-tree for OpenMP ALLOCATE clause

Fortran: Fix parse-dump-tree for OpenMP ALLOCATE clause

Commit r14-1301-gd64e8e1224708e added u2.allocator to gfc_omp_namelist
for better readability and to permit to use namelist->expr for code
like the following:
  !$omp allocators allocate(align(32) : dt%alloc_comp)
    allocate (dt%alloc_comp(5))
  !$omp allocate(dt%alloc_comp2) align(64)
    allocate (dt%alloc_comp2(10))
However, for the parse-tree dump the change was incomplete.

gcc/fortran/ChangeLog:

	* dump-parse-tree.cc (show_omp_namelist): Fix dump of the allocator
	modifier of OMP_LIST_ALLOCATE.