atomics: update llvmcall to use opaque pointers

[inkydragon]

Changes:

• Use ptr for opaque pointers with llvmcall
• Need Julia 1.11+:
  We need LLVM 15+ for pointer type ptr and Julia 1.11+ for opaque-pointers mode.
  See: Pointer Type - LLVM 15;
  Bump Julia to LLVM 16 #51720
• 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.
  Or I can copy build matrix from ci.yml if you want.

TODO:

• Update CI, also need Bump codecov/codecov-action from 3 to 5 #52
• Set version to v"0.6.0"?