RFR: 8075635 - Remove GenerationSpec array

[Jesper Wilhelmsson]

Hi,

Please review another follow up after the generation array removal. In this 
change the array of generation specifications is reduced to two variables in the 
same way as was done for the generations.

The reference to the genSpecs that was previously duplicated in GenCollectedHeap 
is now only available through the collector policy class.

Bug: https://bugs.openjdk.java.net/browse/JDK-8075635
Webrev: http://cr.openjdk.java.net/~jwilhelm/8075635/webrev.00

Thanks!
/Jesper