RFR: 8218743: SA: Add support for large bitmaps

[Stefan Karlsson]

Hi all,

Please review this patch to add support for large bitmaps in the SA.

http://cr.openjdk.java.net/~stefank/8218743/webrev.01/
https://bugs.openjdk.java.net/browse/JDK-8218743

I've added a minimal interface (at, atPut, clear) that uses longs 
instead of ints, and changed MarkBits to use this interface.

I've implemented a segmented bitmap that, currently, all GCs use when 
asked for a bitmap in MarkBits. Later ZGC will provide its own 
discontiguous bitmap, implementing the new interface, but that will be 
sent out as a separate RFE.

I've tested this with a temporary patch that lowers the segment size and 
implements a jtreg test:
http://cr.openjdk.java.net/~stefank/8218743/webrev.test.01

I ran through the all the tests in serviceability/sa with both these 
patches.

Thanks,
StefanK