atomics: update llvmcall to use opaque pointers

[inkydragon]

Close #51

Changes:

• Use ptr for opaque pointers with llvmcall
• Add compat for Julia 1.11+
• Add comment for atomicrmw_ops
• CI: Update build matrix:
  ref: https://github.com/JuliaLang/Pkg.jl/blob/8d3cf02e5e35913c89440c3b5c6678c1a806e975/.github/workflows/test.yml#L28-L49
• CI: run with default arch with julia nightly.
• CI: Update codecov-action.
  Replace Bump codecov/codecov-action from 3 to 5 #52

CI test matrix summary

Nightly CI

• [macOS-latest, ubuntu-latest, windows-latest] * default arch * [v"nightly"]
• and (ubuntu-latest, x64, v"nightly"; exclusive=1, threads=2)

Stable CI

• [ubuntu-latest, windows-latest] * [x86, x64] * [v"1.5", v"1"]
• and [macos-13] * [x64] * [v"1.5", v"1"]
• and (macos-latest, aarch64, v"1")
• and (ubuntu-latest, x64, v"1"; exclusive=1, threads=2)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.18%. Comparing base (cb9cf25) to head (305909d).
Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #53      +/-   ##
==========================================
- Coverage   99.20%   96.18%   -3.03%     
==========================================
  Files           4        4              
  Lines         126      131       +5     
==========================================
+ Hits          125      126       +1     
- Misses          1        5       +4     

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[inkydragon]

This pr is ready to be merged.
I've tested with Julia v1.5 ~ v"1.13.0-DEV.717" on Ubuntu.

[DilumAluthge]

Thanks so much @inkydragon!