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.