libstdc++: Reduce default test timeout to 360 seconds

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.