Dont internalize extern definitions and add proper section info

[altugbozkurt07]

As per our discussion with @alessandrod , i added a small fix to make sure extern declarations preserve linkage/calling-convention and add the proper section info to hint the llvm backend for btf datasec generation for ksyms.

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This change is 

[vadorovsky]

Nice work and thanks for adding the compiler test! Just couple of nits and after that I think it's good to go. 🙂

@vadorovsky reviewed all commit messages.
Reviewable status: 0 of 2 files reviewed, 3 unresolved discussions

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-- commits line 2 at r1:
nit: s/Dont/Don't/

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src/llvm/mod.rs line 270 at r1 (raw file):

            if num_blocks == 0 {
                unsafe { LLVMSetSection(value, c".ksyms".as_ptr()) };
                info!(

In DISanitizer (https://github.com/aya-rs/bpf-linker/blob/main/src/llvm/di.rs) we seem to use mostly trace! log level for fixups, let's use it here too instead of info!.

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src/llvm/mod.rs line 277 at r1 (raw file):

            }
        }
        if unsafe { !LLVMIsAGlobalVariable(value).is_null() } {

As clippy suggests, you can merge these two if stamements into one.

[alessandrod]

let's not merge this until we have kfuncs working e2e in aya since this might not be enough