RFR: Move command line flags to separate file

[Per Liden]

This patch moves the ZGC related command line flags into a separate file.

(It's not pretty, but this is the mechanism hotspot offers today. 
Overhauling this mechanism, so that adding a file with some command line 
flags doesn't also involve adding a bunch of macro crap to 5 other 
files, is on the runtime team's backlog/wish-list.)

http://cr.openjdk.java.net/~pliden/zgc/flags_in_separate_file/webrev.0/

/Per