RFR: JDK-8302820: Remove costs for NMTPreInit when NMT is off

[Thomas Stuefe]

NMTPreInit has been brought into question lately (see [JDK-8299196](https://bugs.openjdk.org/browse/JDK-8299196) and [JDK-8301811](https://bugs.openjdk.org/browse/JDK-8301811)). The points of contention were costs paid when NMT is off, and complexity.

I believe NMTPreInit is vital (for reasons why see discussion under [8299196](https://bugs.openjdk.org/browse/JDK-8299196) ), and removing it would be a severe mistake. So let's address the cost problem.

NMTPreInit, in its current form, incurs costs post-init for lookup table lookup to identify pre-init allocations. Granted, this cost is already pretty low since the load factor of that table is small. But we can avoid that lookup completely by allocating pre-init blocks without malloc headers.

That has two advantages:
- costs for NMTPreInit for NMT=off are practically nil now: all that remains is querying NMT tracking level to see if we are pre- or post-init, and we need to do that anyway to see if NMT is switched on. That cost is not going away unless we get rid of NMT itself.
- We can delete the lookup table if NMT is off, since we don't need it nomore, and regain 63352 bytes of memory.

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I have done my best to come up with a good compromise between complexity, startup speed, and memory consumption. With a bit more complexity, penalties to startup speed could be even more reduced (e.g. by shepherding preallocation headers into their arena). 

But I'm between a rock and a hard place here: more complexity increases the chance of "its too complex, let's just remove it", which is a tiny bit stressful tbh. And the one point I feel strongly about is that getting rid of NMTPreInit would be a grave mistake. I also don't think this code needs more optimization.

(Please note that I enter vacation and won't be able to react promptly to reviews.)

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Tests: 
- Manually tested linux x64 and x86 (gtests with all NMT permutations; runtime/NMT)
- GHAs ongoing

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Commit messages:
 - make-nmt-preinit-cheaper-for-nmt-off

Changes: https://git.openjdk.org/jdk/pull/12642/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12642&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8302820
  Stats: 194 lines in 3 files changed: 99 ins; 46 del; 49 mod
  Patch: https://git.openjdk.org/jdk/pull/12642.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12642/head:pull/12642

PR: https://git.openjdk.org/jdk/pull/12642