RFR: 8346005: Parallel: Incorrect page size calculation with UseLargePages

[Albert Mingkun Yang]

Refactor the heap-space and OS memory interface code to clearly separate two related but distinct concepts: `alignment` and `os-page-size`. These are now represented as two fields in `PSVirtualSpace`.

The parallel heap consists of four spaces: old, eden, from, and to. The first belongs to the old generation, while the latter three belong to the young generation.

The size of any space is always aligned to `alignment`, which also determines the unit for resizing. To keep the implementation simple while allowing flexible per-space commit and uncommit operations, each space must contain at least one OS page. As a result, `alignment` is always greater than or equal to `os-page-size`.

When using explicit large pages -- which require pre-allocating large pages before the VM starts -- the actual OS page size is not known until the heap has been reserved. The additional logic in `ParallelScavengeHeap::initialize` detects the OS page size in use and adjusts `alignment` if necessary.

Test: tier1–8

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Commit messages:
 - pgc-largepage

Changes: https://git.openjdk.org/jdk/pull/26700/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=26700&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8346005
  Stats: 230 lines in 15 files changed: 82 ins; 91 del; 57 mod
  Patch: https://git.openjdk.org/jdk/pull/26700.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/26700/head:pull/26700

PR: https://git.openjdk.org/jdk/pull/26700