Return old value instead of void for atomic operations

[grospelliergilles]

Implement #1788.

[codecov]

Codecov Report

Attention: Patch coverage is 87.50000% with 2 lines in your changes missing coverage. Please review.

Project coverage is 70.24%. Comparing base (17420b3) to head (6501665).
Report is 4 commits behind head on main.

✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
...ane/src/arcane/tests/accelerator/AtomicUnitTest.cc	77.77%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1794      +/-   ##
==========================================
- Coverage   70.24%   70.24%   -0.01%     
==========================================
  Files        2259     2259              
  Lines      162374   162383       +9     
  Branches    18704    18705       +1     
==========================================
+ Hits       114057   114062       +5     
- Misses      41543    41548       +5     
+ Partials     6774     6773       -1     

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