RFR: 8255455: Refactor ThreadHeapSampler::_log_table as constexpr

[Claes Redestad]

The static `ThreadHeapSampler::_log_table` is currently initialized on JVM bootstrap to an overhead of ~67k instructions (linux-x64). By turning the initialization into a constexpr, we can precalculate the helper table at compile time, which trades a runtime overhead for a small, 8kb, static footprint increase.

I compared `fast_log2` with the `log2` builtin with a naive benchmarking experiment[1] (not included in this PR) and show that the `fast_log2` is ~2.5x faster than `log2` on my system. And that without the lookup table we'd be much worse. So I think it makes sense to preserve this optimization, but get rid of the startup overhead:

[5.428s][debug][heapsampling] log2, 0.0751173 secs
[5.457s][debug][heapsampling] fast_log2, 0.0298244 secs
[5.622s][debug][heapsampling] fast_log2_uncached, 0.1645569 secs

I've verified that this refactoring does not affect performance in this naive setup.

[1] https://github.com/openjdk/jdk/compare/master...cl4es:log2_micro?expand=1

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Commit messages:
 - Refactor ThreadHeapSampler::_log_table as constexpr

Changes: https://git.openjdk.java.net/jdk/pull/880/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=880&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8255455
  Stats: 23 lines in 2 files changed: 7 ins; 6 del; 10 mod
  Patch: https://git.openjdk.java.net/jdk/pull/880.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/880/head:pull/880

PR: https://git.openjdk.java.net/jdk/pull/880