RFR: 8264540: WhiteBox.metaspaceReserveAlignment should return shared region alignment [v2]

[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

Yumin Qi has updated the pull request incrementally with one additional commit since the last revision:

  Fix minimal build failure

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/3309/files
  - new: https://git.openjdk.java.net/jdk/pull/3309/files/c8d80a05..f99b1c90

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=3309&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=3309&range=00-01

  Stats: 6 lines in 1 file changed: 6 ins; 0 del; 0 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