Fix object file generation

[learnforpractice]

Make sure object file generation command like below works:

codon build --obj --march=wasm32 -o a.o a.codon

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[arshajii]

Thanks a lot for the PR -- this looks good to me. Can we please do the following:

• Since we're moving the llvm/CodeGen/CommandFlags.h include to llvm.h, can we delete it from the following files: codon/cir/llvm/gpu.cpp, codon/cir/llvm/llvisitor.cpp and codon/cir/llvm/optimize.cpp. (There used to be some issue with this include in older LLVM versions, which is why it was done this way originally.)

• Run clang-format (i.e. clang-format --style=file -i <changed files>) (should alphabetize llvm.h includes)