openacc: Fix atomic_capture-2.c iteration-ordering issues

The test case was written with assumptions about loop iteration ordering
that are not guaranteed by OpenACC and do not apply on all targets,
in particular AMD GCN. This patch removes those assumptions.

2020-09-08  Julian Brown  <julian@codesourcery.com>

libgomp/
	* testsuite/libgomp.oacc-c-c++-common/atomic_capture-2.c: Remove
	iteration-ordering assumptions.