RFR: 8273872: ZGC: Explicitly use 2M large pages

[Per Liden]

ZGC requires large pages to be 2M. However, ZGC doesn't explicitly asks for this page size and instead relies on the default large pages size for the system to be 2M. On systems where this is not true, ZGC will fails with an error message. To avoid this, ZGC should explicitly ask for 2M large pages and not rely on the system default. Furthermore, ZGC currently ignores `-XX:LargePageSizeInBytes`. ZGC should fails with an error message if it's specified to something other than 2M.

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Commit messages:
 - 8273872: ZGC: Explicitly use 2M large pages

Changes: https://git.openjdk.java.net/jdk/pull/5541/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=5541&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8273872
  Stats: 11 lines in 2 files changed: 9 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5541.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5541/head:pull/5541

PR: https://git.openjdk.java.net/jdk/pull/5541