libstdc++: Increase timeout for tests in debug mode

These tests FAIL when testing debug mode with a small tool_timeout
value. Give them a longer relative timeout.

libstdc++-v3/ChangeLog:

	* testsuite/25_algorithms/lexicographical_compare/deque_iterators/1.cc:
	Add dg-timeout-factor directive.
	* testsuite/ext/pb_ds/regression/tree_map_rand_debug.cc:
	Increase timeout factor from 2.0 to 3.0.
	* testsuite/ext/pb_ds/regression/tree_set_rand_debug.cc:
	Likewise.
	* testsuite/lib/libstdc++.exp (check_effective_target_debug-mode):
	Define "debug-mode" as an effective-target keyword.