RFR: 8080581: Align SA with new GC directory structure

[Per Liden]

Hi,

This is a follow up patch to the GC directory restructure change, which 
aligns the remaining part of the SA with the new GC structure.

The patch moves some GC-related files into new locations and updates 
package and import lines.

Webrev: http://cr.openjdk.java.net/~pliden/8080581/webrev.0/
Bug: https://bugs.openjdk.java.net/browse/JDK-8080581

cheers,
/Per