Conflicts exist if move to jdk14

[Jie He]

Hi 

I had a quick try to rebase jdk/tsan to jdk14-ga, and found 2 major conflicts:
1, Shenandoah gc had a refactor last year, which implements a concurrent evacuation, seems it impacts current tsan oop map.
2, they limited non-gc to access the reserved region in gc, adding the function back to collectedHeap is a workround.

In addition, a known build error exists in AARCH64 when building openjdk release version with clang 8. changing the -O3 to -Os or -O0 will be ok.

An initial test shows 77 passed of total 79 tsan cases. The failed cases are NonRacyFinalizerLoopTest.java and tsan/NonRacyGarbageCollectionLoopTest.java.

Thanks
Jie He