GCN: Wait for exit value to write before exiting.

2019-05-24  Andrew Stubbs  <ams@codesourcery.com>

	gcc/
	* config/gcn/gcn-run.c (main): Set a non-zero return value if the
	kernel does not exit cleanly.
	* config/gcn/gcn.md (gcn_return): Insert s_waitcnt before s_dcache_wb.

From-SVN: r271605