marked sunpro-5_3 unusable for iostreams

[SVN r27385]

status/explicit-failures-markup.xml

@@ -474,6 +474,12 @@
 
     <!-- iostreams -->
     <library name="iostreams">
+        <mark-unusable>
+            <toolset name="sunpro-5_3"/>
+            <note author="Jonathan Turkanis">
+		        Many compiler-specific workarounds will have to applied before toolset is supported.
+            </note>
+        </mark-unusable>
         <mark-expected-failures>
             <test name="seekable_file_test"/>
 			<toolset name="gcc-2.95.3-stlport-4.5.3-linux"/>