Fix OpenACC/GCN 'acc_ev_enqueue_launch_end' position

For an OpenACC compute construct, we've currently got:

  - [...]
  - acc_ev_enqueue_launch_start
  - launch kernel
  - free memory
  - acc_ev_free
  - acc_ev_enqueue_launch_end

This confused another thing that I'm working on, so I adjusted that to:

  - [...]
  - acc_ev_enqueue_launch_start
  - launch kernel
  - acc_ev_enqueue_launch_end
  - free memory
  - acc_ev_free

Correspondingly, verify 'acc_ev_alloc', 'acc_ev_free' in
'libgomp.oacc-c-c++-common/acc_prof-parallel-1.c'.

	libgomp/
	* plugin/plugin-gcn.c (gcn_exec): Fix 'acc_ev_enqueue_launch_end'
	position.
	* testsuite/libgomp.oacc-c-c++-common/acc_prof-parallel-1.c:
	Verify 'acc_ev_alloc', 'acc_ev_free'.