gccgo: link static libgo against -lrt on GNU/Linux

gccgo: link static libgo against -lrt on GNU/Linux

The upcoming Go 1.18 release requires linking against -lrt on GNU/Linux
(only) in order to call timer_create and friends.

Also change gotools to link the runtime test against -lrt.

	* gospec.cc (RTLIB, RT_LIBRARY): Define.
	(lang_specific_driver): Add -lrt if linking statically on
	GNU/Linux.

	* configure.ac (RT_LIBS): Define.
	* Makefile.am (check-runtime): Set GOLIBS to $(RT_LIBS).
	* configure, Makefile.in: Regenerate.