perf: Enable THP by default

[orlp]

If a system supports THP (transparent huge pages) but has it set by default to only use it when madvise is called (which is the default on for example Ubuntu server), then we wouldn't use THP before this PR. This PR ensures that jemalloc will madvise every allocation with MADV_HUGEPAGE so that they will be used on such systems.

On PDS-H SF=10 and SF=100 I observed performance increases of 15-20% with the new streaming engine. I expect THP to pretty much always be beneficial for Polars unless we start swapping memory in random access patterns (but arguably when you reach that point you have already lost as an OLAP system).

If for whatever reason you find THP to be a regression, please do let us know, in the meantime you can specify the env variable _RJEM_MALLOC_CONF=thp:never,metadata_thp:never to disable it again.

---

The PR does contain a bit of a questionable linker hack to hide jemalloc's warnings on systems that don't support THP. I hope this doesn't break any builds.

I've switched to setting the jemalloc flags in Python now, right before importing the Polars library. This was the only way I could think of to disable setting the THP flag for non-linux systems, as that causes jemalloc to print warnings. I failed in trying to silence these warnings.

[codecov]

Codecov Report

Attention: Patch coverage is 75.00000% with 2 lines in your changes missing coverage. Please review.

Project coverage is 80.94%. Comparing base (6fd6e01) to head (7a3d86d).
Report is 7 commits behind head on main.

Files with missing lines	Patch %	Lines
py-polars/polars/__init__.py	75.00%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #21829      +/-   ##
==========================================
- Coverage   81.00%   80.94%   -0.06%     
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  Files        1620     1623       +3     
  Lines      233617   233988     +371     
  Branches     2691     2693       +2     
==========================================
+ Hits       189249   189413     +164     
- Misses      43738    43943     +205     
- Partials      630      632       +2     

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