c++: contracts fixes

Fixing -Wunused-parm warnings and link errors depending on where -fcontracts
appears on the command line.

gcc/cp/ChangeLog:

	* contracts.cc (build_contract_condition_function):
	Set DECL_ARTIFICIAL on return value parm.
	* g++spec.cc (lang_specific_driver): Add -lstdc++exp
	just before -lstdc++.