RFR: 8264540: WhiteBox.metaspaceReserveAlignment should return shared region alignment

[Yumin Qi]

Hi, Please review
  After JDK-8236847, the shared region alignment (new as MetaspaceShared::core_region_alignment) is no longer default to os pagesize, it is a configurable value at build time instead. The WhiteBox api metaspaceReserveAlignment() should reflect the change. 

Tests:tier1,tier2,tier3,tier4

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Commit messages:
 - 8264540: WhiteBox.metaspaceReserveAlignment should return shared region alignment

Changes: https://git.openjdk.java.net/jdk/pull/3309/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=3309&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8264540
  Stats: 23 lines in 5 files changed: 8 ins; 3 del; 12 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3309.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3309/head:pull/3309

PR: https://git.openjdk.java.net/jdk/pull/3309