[llvm] [BPF] Avoid relocation for jumptable entries (PR #166301)

[via llvm-commits]

yonghong-song wrote:

@eddyz87 has similar idea before (to have (label - <start_section>)/8 as the jump table entry). Later on it is decided that label itself is good enough. This patch merely change the jump table entry to (label - <start_section>) to avoid unnecessary relocation for jump table.

https://github.com/llvm/llvm-project/pull/166301