bpf: add documentation to compare clang "-target bpf" and default target

   By the way, the BPF kernel selftests run with -mcpu=probe for better

   test coverage.

Q: In some cases clang flag "-target bpf" is used but in other cases the

   default clang target, which matches the underlying architecture, is used.

   What is the difference and when I should use which?

A: Although LLVM IR generation and optimization try to stay architecture

   independent, "-target <arch>" still has some impact on generated code:

     - BPF program may recursively include header file(s) with file scope

       inline assembly codes. The default target can handle this well,

       while bpf target may fail if bpf backend assembler does not

       understand these assembly codes, which is true in most cases.

     - When compiled without -g, additional elf sections, e.g.,

       .eh_frame and .rela.eh_frame, may be present in the object file

       with default target, but not with bpf target.

     - The default target may turn a C switch statement into a switch table

       lookup and jump operation. Since the switch table is placed

       in the global readonly section, the bpf program will fail to load.

       The bpf target does not support switch table optimization.

       The clang option "-fno-jump-tables" can be used to disable

       switch table generation.

   You should use default target when:

     - Your program includes a header file, e.g., ptrace.h, which eventually

       pulls in some header files containing file scope host assembly codes.

     - You can add "-fno-jump-tables" to work around the switch table issue.

   Otherwise, you can use bpf target.

Happy BPF hacking!