Add more explanation in doc/HACKING about how to read gcov output,
including a reference to the gcov documentation in the GCC manual.
Also add details about how our postprocessing scripts modify gcov
output.
Document this convention.
Add a script to post-process .gcov files in order to stop nagging us
about excluded lines.
Teach cov-diff to handle these post-processed files.
Closes ticket 16792