RFR: 8302043 : [AIX] Safefetch fails for bad_addressN and bad_address32

[Varada M]

On AIX, reading from lower range addresses like 5120(5K) does not cause a segmentation fault, though writing to them does. Hence the safefetch tests don't run as expected on AIX. This is solved by setting bad_addressN and bad_address32 to -1 for AIX. 
These tests under NMTGtests.java also passes :
gtest/NMTGtests.java#nmt-detail
gtest/NMTGtests.java#nmt-summary
gtest/NMTGtests.java#nmt-off

Reported issue: [JDK-8302043](https://bugs.openjdk.org/browse/JDK-8302043)

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Commit messages:
 - safefetch for bad address

Changes: https://git.openjdk.org/jdk/pull/12468/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12468&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8302043
  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/12468.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12468/head:pull/12468

PR: https://git.openjdk.org/jdk/pull/12468