runtime: call timer functions via syscall

It turns out to be painful to require linking against -lrt on
GNU/Linux, as that makes it harder to link Go code into C programs.
Instead just call the timer syscalls directly.  That is what the
upstream library does anyhow.

gcc/go/
	* gospec.cc: Revert 2022-02-09 change:
	(RTLIB, RT_LIBRARY): Don't define.
	(lang_specific_driver): Don't add -lrt if linking statically
	on GNU/Linux.

gotools/
	* configure.ac: Revert 2022-02-09 change:
	(RT_LIBS): Don't define.
	* Makefile.am (check-runtime): Don't set GOLIBS to $(RT_LIBS).
	* configure, Makefile.in: Regenerate.

Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/385475