feat: More memmaps through semaphore

[ritchie46]

Also moved the emscripten snippet there, so more mmap's will succeed.

[codecov]

Codecov Report

Attention: Patch coverage is 50.00000% with 2 lines in your changes missing coverage. Please review.

Project coverage is 79.00%. Comparing base (5294cb5) to head (4e130dd).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/polars-io/src/ipc/ipc_reader_async.rs	0.00%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #20423   +/-   ##
=======================================
  Coverage   79.00%   79.00%           
=======================================
  Files        1562     1562           
  Lines      220166   220166           
  Branches     2486     2486           
=======================================
+ Hits       173944   173950    +6     
+ Misses      45649    45643    -6     
  Partials      573      573           

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