[OpenACC] Allow gang reductions inside serial constructs

[OpenACC] Allow gang reductions inside serial constructs

... fixing a regression introduced in the preceding
commit 2b7dac2c0dcb087da9e4018943c023c0678234a3
"Make OpenACC orphan gang reductions errors".

	gcc/fortran/
	* openmp.c (oacc_is_serial, oacc_is_parallel_or_serial): New.
	(resolve_oacc_loop_blocks): Use oacc_is_parallel_or_serial instead of
	oacc_is_parallel.
	libgomp/
	* testsuite/libgomp.oacc-fortran/parallel-dims.f90: Remove
	temporary skip.

Co-Authored-By: Thomas Schwinge <thomas@codesourcery.com>