gcov: Add __gcov_filename_to_gcfn()

gcov: Add __gcov_filename_to_gcfn()

gcc/

	* doc/invoke.texi (fprofile-info-section): Mention
	__gcov_filename_to_gcfn().  Use "freestanding" to match with C11
	standard language.  Fix minor example code issues.
	* gcov-io.h (GCOV_FILENAME_MAGIC): Define and document.

gcc/testsuite/

	* gcc.dg/gcov-info-to-gcda.c: Test __gcov_filename_to_gcfn().

libgcc/

	* gcov.h (__gcov_info_to_gcda): Mention __gcov_filename_to_gcfn().
	(__gcov_filename_to_gcfn): Declare and document.
	* libgcov-driver.c (dump_string): New.
	(__gcov_filename_to_gcfn): Likewise.
	(__gcov_info_to_gcda): Adjust comment to match C11 standard language.