bpf, doc: add description wrt native/bpf clang target and pointer size

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

       switch table generation.

     - For clang -target bpf, it is guaranteed that pointer or long /

       unsigned long types will always have a width of 64 bit, no matter

       whether underlying clang binary or default target (or kernel) is

       32 bit. However, when native clang target is used, then it will

       compile these types based on the underlying architecture's conventions,

       meaning in case of 32 bit architecture, pointer or long / unsigned

       long types e.g. in BPF context structure will have width of 32 bit

       while the BPF LLVM back end still operates in 64 bit. The native

       target is mostly needed in tracing for the case of walking pt_regs

       or other kernel structures where CPU's register width matters.

       Otherwise, clang -target bpf is generally recommended.

   You should use default target when:

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