RFR: 8205676: ZGC: Remove TLAB allocations in relocation path

[Per Liden]

During Java-thread aided relocation, ZGC tries to use the TLAB to 
allocate the new object. However, this interacts badly with JEP 331: 
Low-Overhead Heap Profiling, as it distorts the profiling statistics. I 
propose we remove the use of the TALB in the relocation path, 
essentially changing a thread-local pointer bump to a uncontended 
CPU-local CAS.

Bug: https://bugs.openjdk.java.net/browse/JDK-8205676
Webrev: http://cr.openjdk.java.net/~pliden/8205676/webrev.0

Testing: Currently running benchmarks to verify that this has no 
unexpected performance hit.

I've filed two follow up RFEs (also out for review now), to clean up 
some functions that are now unused:
https://bugs.openjdk.java.net/browse/JDK-8205678
https://bugs.openjdk.java.net/browse/JDK-8205679

/Per