Document -march=gfx906 option.

2019-06-07  Andrew Stubbs  <ams@codesourcery.com>

	gcc/
	* doc/invoke.texi (AMD GCN Options): Add gfx906.

From-SVN: r272044

gcc/ChangeLog

+2019-06-07  Andrew Stubbs  <ams@codesourcery.com>
+
+	* doc/invoke.texi (AMD GCN Options): Add gfx906.
+
 2019-06-07  Richard Biener  <rguenther@suse.de>
 
 	PR debug/90574

gcc/doc/invoke.texi

@@ -16211,6 +16211,9 @@ Compile for GCN3 Fiji devices (gfx803).
 @item gfx900
 Compile for GCN5 Vega 10 devices (gfx900).
 
+@item gfx906
+Compile for GCN5 Vega 20 devices (gfx906).
+
 @end table
 
 @item -mstack-size=@var{bytes}