RFR: JDK-8033236 Update GensrcCharsetMapping.gmk to build-infra standards

[Magnus Ihse Bursie]

The file GensrcCharsetMapping.gmk was not properly converted from the 
old build system.

This leads to unneccessary output, incorrect dependencies, missed 
chances for parallelization. It also breaks certain attempts to run 
static code analyzers such as Parfait.

This change introduces the new API TextFileProcessing into jdk9. It's 
been in the build-infra project for a while. Similar to the compile 
constructs, we can setup a file (or a bunch of files) which needs 
different kinds of text transformations. In this case, only the INCLUDE 
functionality is used.

This is also the first in hopefully a series of patches that will try to 
bring some order into the mess of generated/processed files. The 
following rules have been used:
* Files that are almost java source files, but needs some text 
processing, should be named "Foo.java.template" and live in the correct 
place in src/classes.
* Files that provides data to any gensrc action, but that are not at all 
java source code, should live in make/data/<descriptive dir>.

Bug: https://bugs.openjdk.java.net/browse/JDK-8033236
WebRev: 
http://cr.openjdk.java.net/~ihse/JDK-8033236-update-gensrc-charset-mapping-gmk/webrev.01/

/Magnus