RFR: 8221212: ZGC: Command-line flags should be marked experimental

[Per Liden]

ZGC (the -XX:+UseZGC command-line flag) is marked experimental, which 
means that you always need to use -XX:+UnlockExperimentalVMOptions to 
unlock that option. However, some of ZGC's flags are incorrectly marked 
as product flags. This can be a bit confusing, since those flags (but 
not the -XX:+UseZGC flag) will show up when using -XX:PrintFlagsFinal, 
even though they are effectively experimental and has no use/meaning 
unless ZGC is enabled.

We should not mark any ZGC flags as product flags, until the -XX:+UseZGC 
flag is made a product flag.

I have not filed a CSR for this, since these flags are effectively 
experimental already today, but just aren't properly marked as such.

Bug: https://bugs.openjdk.java.net/browse/JDK-8221212
Webrev: http://cr.openjdk.java.net/~pliden/8221212/webrev.0

/Per