libstdc++: Reduce default test timeout to 360 seconds

The current default of 10 minutes is much longer than most tests need on
common hardware. The slow tests all now have a dg-timeout-factor
directive that gives them more time to run relative to the default. The
default can also be overridden in ~/.dejagnurc or DEJAGNU=site.exp, so
it seems unnecessary to have such a large default.

This reduces the default from 10 minutes to 6 minutes, which still seems
more than enough.

libstdc++-v3/ChangeLog:

	* testsuite/lib/libstdc++.exp (libstdc++_init): Reduce
	default tool_timeout to 360.

libstdc++-v3/testsuite/lib/libstdc++.exp

@@ -248,7 +248,7 @@ proc libstdc++_init { testfile } {
     # Set the default timeout for v3 tests.
     # You can override this in ~/.dejagnurc or a .exp file named by $DEJAGNU.
     if {![info exists tool_timeout]} {
-      set tool_timeout 600
+      set tool_timeout 360
     }
 
     # Default settings.