RFR: 8024137 - Flags should be set using the proper macro

[Jesper Wilhelmsson]

Hi,

Please review this change to make the GC ergonomics use FLAG_SET_ERGO() instead 
of setting values directly in flags. This change builds on the fix for 
JDK-8048093 and assumes that we will remember that the flags was set on the 
command line.

If you know of any other places where we assign a flag directly without using 
FLAG_SET_ERGO() please let me know since it would make sense to change all these 
places in this change.

Bug: https://bugs.openjdk.java.net/browse/JDK-8024137
Webrev: http://cr.openjdk.java.net/~jwilhelm/8024137/webrev.00/

Thanks,
/Jesper