RFR: JDK-8303817: Add constexpr for natural malloc alignment

Wed Mar 8 11:40:01 UTC 2023

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I miss having an easy way to get the alignment the libc guarantees for malloc. Let's add this, and we can use this right away with NMT's MallocHeader.

Tests: manually built and tested x64 linux (clang + GCC) and x86 linux.

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Commit messages:
 - JDK-8303817-constexpr-for-natural-malloc-alignment

Changes: https://git.openjdk.org/jdk/pull/12921/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12921&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8303817
  Stats: 19 lines in 3 files changed: 17 ins; 1 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/12921.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12921/head:pull/12921

PR: https://git.openjdk.org/jdk/pull/12921