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

[Erik Joelsson]

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

/Erik