RFR: JDK-8171922: Hotspot code coverage doesn't seem to work

[Tim Bell]

Erik:

> When the new hotspot build was introduced, we missed converting the
> support for compiling in code coverage using gcov. This patch fixes
> that. While making the change I discovered that the LEGACY_EXTRA_*FLAGS
> variables are now unused and can finally be completely removed, so I
> went ahead and did that too.
>
> Bug: https://bugs.openjdk.java.net/browse/JDK-8171922
>
> Webrev: http://cr.openjdk.java.net/~erikj/8171922/webrev.01/

Looks good to me.

/Tim