This allows the default timeout for libstdc++ tests to be set by the
user, either in ~/.dejagnurc or a site.exp file that $DEJAGNU names.

libstdc++-v3/ChangeLog:

	* testsuite/lib/libstdc++.exp (libstdc++_init): Only set
	tool_timeout if it hasn't been set by the user already.

Tested powerpc64le-linux. Committed to trunk.