[llvm] [BPF] Use ".L" local prefix label for basic blocks (PR #95103)

[Fangrui Song via llvm-commits]

MaskRay wrote:

> Okay, looks like current llvm19/llvm20 already do not have L* labels in symbol table. @eddyz87 has another pull request (#100550) to address llvm-objdump without-label issue. There are a couple of options to resolve this. Since we do not want L* in the symbol table, then the only option may be tweak llvm-objdump.

#100550 is for `llvm-objdump --symbolize-operands`, which creates fake labels for the disassembly output.

For this patch, I hope we use `.L` to be similar to other ELF architectures and not to create a special case:)

https://github.com/llvm/llvm-project/pull/95103