RFR: JDK-8239789 Follow-up on JVM feature rewrite

[Magnus Ihse Bursie]

The JVM feature rewrite was actually somewhat of a step backwards in 
terms of presenting information for the user what different configure 
arguments do.

I've compensated for this by making it far better! :) Now we get a list 
like this:

   aot                     enable ahead of time compilation (AOT)
   cds                     enable class data sharing (CDS)
   compiler1               enable hotspot compiler C1
   compiler2               enable hotspot compiler C2
   dtrace                  enable dtrace support
   epsilongc               include the epsilon (no-op) garbage collector
   g1gc                    include the G1 garbage collector
   graal                   enable Graal (jdk.internal.vm.compiler)
   jfr                     enable Java Flight Recorder (JFR)
   jni-check               enable -Xcheck:jni support
   jvmci                   enable JVM Compiler Interface (JVMCI)
   jvmti                   enable Java Virtual Machine Tool Interface 
(JVM TI)
   link-time-opt           enable link time optimization
   management              enable java.lang.management API support
   minimal                 support building variant 'minimal'
   nmt                     include native memory tracking (NMT)
   parallelgc              include the parallel garbage collector
   serialgc                include the serial garbage collector
   services                enable diagnostic services and client attaching
   shenandoahgc            include the Shenandoah garbage collector
   static-build            build static library instead of dynamic
   vm-structs              export JVM structures to the Serviceablility 
Agent
   zero                    support building variant 'zero'
   zgc                     include the ZGC garbage collector

Also, I missed using the recommended m4_ prefix on some m4 functions. 
This patch also corrects this.

Bug: https://bugs.openjdk.java.net/browse/JDK-8239789
WebRev: 
http://cr.openjdk.java.net/~ihse/JDK-8239789-jvm-features-followup/webrev.01

/Magnus