RFR(S): 8195691: AIX build broken after 8194312

[Doerr, Martin]

Hi Kim,

8194312 introduced 2 build problems for xlC (version 12.01.0000.0019):
1. test/hotspot/gtest/gc/shared/test_oopStorage.cpp", line 1196.25: 1540-0300 (S) The "private" member "class OopStorage::BlockList" cannot be accessed.
2. test/fmw/gtest/include/gtest/gtest.h", line 1448.7: 1540-0207 (S) No common type found for operands with type "long" and "OopStorage::Block *".

I have found a way to get it working again. Please review:
http://cr.openjdk.java.net/~mdoerr/8195691_fix_AIX_build/webrev.00/

For 1., I have made the classes public for AIX only.
For 2., I have introduced NULL_BLOCK to help the compiler to derive the type.

Best regards,
Martin