[gcov] Add -i --intermediate-format

Between gcov 4.9~8, `gcov -i $file` prints coverage information to
$file.gcov in an intermediate text format (single file, instead of
$source.gcov for each source file).

lcov newer than 2019-05-24 detects -i support and uses it to increase
processing speed.  gcov 9 (GCC r265587) removed --intermediate-format
and -i was changed to mean --json-format. However, we consider this
format still useful and support it. geninfo (part of lcov) supports this
format even if we announce that we are compatible with gcov 9.0.0