gcov: dump in a static dtor instead of in an atexit handler

	PR gcov-profile/7970
	PR gcov-profile/16855
	PR gcov-profile/44779
	* g++.dg/gcov/pr16855.C: New test.
	* coverage.c (build_gcov_exit_decl): New function.
	(coverage_obj_init): Call the function and generate __gcov_exit
	destructor.
	* doc/gcov.texi: Document when __gcov_exit function is called.
	* libgcov-driver.c (__gcov_init): Do not register a atexit
	handler.
	(__gcov_exit): Rename from gcov_exit.
	* libgcov.h (__gcov_exit): Declare.

From-SVN: r240529