RFR: JDK-8221762: Improve Main.gmk/FindTests.gmk bootstrap time

[Tim Bell]

Erik:

> With the new run-test test running framework, we introduced
> FindTests.gmk, which among other things, parses all the jtreg
> TEST.groups files to generate make targets for each group. While this is
> very nice, the current implementation in FindTests.gmk adds significant
> time to a simple "do nothing" make execution.
>
> I have experimented with a different implementation, based on caching
> the expensive calculations in a makefile that automatically gets
> re-generated if any of the inputs change.
>
> On my Linux workstation, running the very early make target
> "buildtools-langtools" with the old and new implementation gives the
> following times (real time as reported by the time command):
>
> Old: 460-470ms
> New (warm cache): 280-285ms
> New (cold cache): 370-380ms
>
> So, even with the cache cold, the new implementation is faster, and with
> the warm cache, 180ms is quite a lot. It can definitely be felt when
> running interactively in the shell.
>
> The new implementation also removes the need for the workaround in
> InitSupport.gmk introduced in JDK-8213736, so I'm removing that.
>
> Bug: https://bugs.openjdk.java.net/browse/JDK-8221762
>
> Webrev: http://cr.openjdk.java.net/~erikj/8221762/webrev.01/index.html


Looks good.

Tim