RFR: JDK-8217728: Speed up incremental rerun of "make hotspot"

[Tim Bell]

Erik:

> This patch improves general incremental build performance of native
> binaries, which in particular improves performance for rebuilding
> hotspot. Specifically it removes overhead in the make file processing.
> Two major issues are addressed:
>
> 1. Long ago, I had managed to introduce an O(n^2) construct when keeping
> track of which object files have already been processed. This is used to
> support overriding shared files with platform specific (or open/custom).
> Basically the first file with a specific name takes precedence over any
> other files with the same name. Changed this to a simple variable lookup.
>
> 2. For each compilation unit, we have the compiler generate make
> dependency files to help correctly build incrementally on the next make
> invocation. When there are many compilation units, there will be a lot
> of these files, so opening and parsing each file takes time. I've added
> an additional step where all of these file are concatenated at the end
> so that the next make invocation only needs to open one single file
> (with a fallback to reading the individual files for robustness).
> Combining the files also allows for removing a ton of duplicate
> information with "sort -u".
>
> I'm also fixing the hotspot-only and hotspot-<variant>-only top level
> targets (in Main.gmk) which I discovered weren't functioning while
> testing the changes.
>
> Bug: https://bugs.openjdk.java.net/browse/JDK-8217728
>
> Webrev: http://cr.openjdk.java.net/~erikj/8217728/webrev.01

Looks good.

Tim