RFR: 8253332: ZGC: Make heap views reservation platform independent

[Stefan Karlsson]

ZVirtualMemoryManager::reserve_contiguous_platform tries to reserve three views of a given address range. The posix and
windows versions are more or less duplicates, with calls to platform dependent versions of reserve/unreserve functions.

I'd like to clean this up in preparation of an alternative implementation for heap memory allocation on Windows.

I choose to prefix the OS dependent functions with os_. For consistency, initialize_os should have been renamed as
well, but the plan is to change that in a separate patch that splits that function into two, so I skipped it for now.

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Commit messages:
 - 8253332: ZGC: Make heap views reservation platform independent

Changes: https://git.openjdk.java.net/jdk/pull/236/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=236&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8253332
  Stats: 129 lines in 4 files changed: 50 ins; 62 del; 17 mod
  Patch: https://git.openjdk.java.net/jdk/pull/236.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/236/head:pull/236

PR: https://git.openjdk.java.net/jdk/pull/236