BPF: generate BTF info for LD_imm64 loaded function pointer

For an example like below,
    extern int do_work(int);
    long bpf_helper(void *callback_fn);
    long prog() {
        return bpf_helper(&do_work);
    }

The final generated codes look like:
    r1 = do_work ll
    call bpf_helper
    exit
where we have debuginfo for do_work() extern function:
    !17 = !DISubprogram(name: "do_work", ...)

This patch implemented to add additional checking
in processing LD_imm64 operands for possible function pointers
so BTF for bpf function do_work() can be properly generated.
The original llvm function name processReloc() is renamed to
processGlobalValue() to better reflect what the function is doing.

Differential Revision: https://reviews.llvm.org/D100568