Please review the change to support dual mode launcher

[Mandy Chung]

Alan,

Can you review this fix?

Webrev at:
  http://cr.openjdk.java.net/~mchung/jigsaw/dual-mode-launcher/

The language tools such as javac, javap, and javah needs to access all 
platform classes.  In legacy world, it will access the platform classes 
from the bootclasspath.   However, in modules world, a module can only 
access classes from its required modules.  For example, javap won't be 
able to access xml module:
   jdk-module-image/bin/javap javax.xml.XMLConstants

So this patch will fix the launcher makefile to build javah and javap to 
run in legacy mode like javac.   When Jon pushes  his patch for javac to 
use the jigsaw resolver to find platform classes, javac could switch to 
run in module mode.   I added an internal switch in the launcher, 
-mode:legacy or -mode:module, to specify a tool to run in legacy mode or 
module mode when running in a modules image (e.g. jdk-module-image). 
This is temporary and only valid for jdk tools to use.

Jon,
   Does javadoc need to access platform classes like javap and javah?   
In your patch, javac uses the jigsaw resolver to find platform 
classes.   How about javah and javap?

Thanks
Mandy