varnish gcov

[Poul-Henning Kamp]

After a lot of comparisons I have changed the GCOV to use LLVM-8.0
because it is about twice as fast and (IMO) more honest than the
GCC6 we used.

This means that the overall score dropped a little bit.

I also rescaled the "sparklines" to make incremental changes in
the range we care about stand out more.

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Poul-Henning Kamp       | UNIX since Zilog Zeus 3.20
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