RFR: JDK-8322943: runtime/CompressedOops/CompressedClassPointers.java fails on AIX [v2]

[Joachim Kern]

> Even after recent fixes like
> https://bugs.openjdk.org/browse/JDK-8305765
> the test runtime/CompressedOops/CompressedClassPointers.java fails on AIX.
> 
> This error results from the fact, that on AIX the shmat() allocation granularity is 256MB instead of the standard Pagesize (4KB or 64KB).
> 
> As a solution we introduce a new method  `os::vm_shm_allocation_granularity()`, which on all platforms except AIX returns the same value as  `os::vm_allocation_granularity()`, but on AIX it returns (in the apropriate cases) 256MB.
> 
> This new getter is used at all needed places instead of  `os::vm_allocation_granularity()`.

Joachim Kern has updated the pull request incrementally with one additional commit since the last revision:

  following Goetz objections

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/17708/files
  - new: https://git.openjdk.org/jdk/pull/17708/files/01f6d47f..dccb54a3

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=17708&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=17708&range=00-01

  Stats: 6 lines in 1 file changed: 0 ins; 5 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/17708.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/17708/head:pull/17708

PR: https://git.openjdk.org/jdk/pull/17708