JDK-8027113 fix code review

[Ron Durbin]

JDK-8027113 decouple the '-XXaltjvm=<path>' option from the gamma launcher

https://bugs.openjdk.java.net/browse/JDK-8027113

Web URL http://cr.openjdk.java.net/~rdurbin/JDK-8027113-webrev/0-hsx25/

Internal URL: 
http://javaweb.us.oracle.com/~rdurbin/JDK-8027113-webrev/0-hsx25/

Summary:
Refactor the code so that it is no longer tightly coupled with the "gamma launcher".
This change is based on a JDK8 hs25 repo, but will be checked into a JDK9 repo when one is ready.

 This will allow work on the following bug to proceed:

    JDK-8005262 possible gamma launcher issues 

Testing included:
 JPRT run
 Manual tests on Solaris 64 bit, Linux 32 bit and 64 bit, WindowsXP and Windows 7.