[OpenACC] Clarify sequencing of 'async' data copying vs. profiling events in 'libgomp...

[OpenACC] Clarify sequencing of 'async' data copying vs. profiling events in 'libgomp.oacc-c-c++-common/acc_prof-{init,parallel}-1.c'

... as noticed with GCN offloading.

Fix-up for r271346 (commit 5fae049dc272144f8e61af94ee0ba42b270915e5)
"OpenACC Profiling Interface (incomplete)".

	libgomp/
	* testsuite/libgomp.oacc-c-c++-common/acc_prof-init-1.c: Clarify
	sequencing of 'async' data copying vs. profiling events.
	* testsuite/libgomp.oacc-c-c++-common/acc_prof-parallel-1.c:
	Likewise.