RFR: 8312180: (bf) MappedMemoryUtils passes incorrect arguments to msync (aix)

[Tyler Steele]

This change is as an alternate solution to my original proposal for [JDK-8312180](https://bugs.openjdk.org/browse/JDK-8312180).

Suggested by @tstuefe.

This change matches the return value of Bits.pageSize with the system page size (which is not currently true on AIX).

These tests are upstream of my target test (TestByteBuffer) and results are encouraging. Tier1 testing in ongoing for this change.

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Commit messages:
 - Replace vm_page_size call with sysconf(_SC_PAGESIZE) in javaClasses.cpp

Changes: https://git.openjdk.org/jdk/pull/14963/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=14963&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8312180
  Stats: 2 lines in 1 file changed: 1 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/14963.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14963/head:pull/14963

PR: https://git.openjdk.org/jdk/pull/14963