add btf func_info and line_info support

[yonghong-song]

The libbpf API functions are used to parse/load
.BTF and .BTF.ext ELF sections, and retrieve
func_info and line_info for prog loading.

The LLVM cannot get remapped file source for
line_info. So a postprocessing after compilation
fixed the line_off (offset to the string table
for the line_info) in .BTF.ext and
adjusted string section in .BTF to accommodate
new source lines.

For users using python and C++ API, no changes
to their codes. If .BTF and .BTF.ext are supported
in both compiler and kernel, they will automatically
get benefit, e.g., verifier log annotated with source
code, bpftool showing source annotated jit codes, etc.

For example, with latest bpf-next and llvm trunk,
running `tcpconnect.py`, bpftool (from kernel) is able
to show the following:

      -bash-4.4$ sudo bpftool p d jited id 165
      int trace_connect_v6_return(struct pt_regs * ctx):
      bpf_prog_8937c3924bd93e28_trace_connect_v6_return:
      ; int trace_connect_v6_return(struct pt_regs *ctx)
         0:   push   %rbp
         1:   mov    %rsp,%rbp
         4:   sub    $0x88,%rsp
         b:   sub    $0x28,%rbp
         f:   mov    %rbx,0x0(%rbp)
        13:   mov    %r13,0x8(%rbp)
        17:   mov    %r14,0x10(%rbp)
        1b:   mov    %r15,0x18(%rbp)
        1f:   xor    %eax,%eax
        21:   mov    %rax,0x20(%rbp)
        25:   mov    %rdi,%rbx
      ; int ret = PT_REGS_RC(ctx);
        28:   mov    0x50(%rbx),%r13
      ; u32 pid = bpf_get_current_pid_tgid();
        2c:   callq  0xffffffffe0e4ebfe
      ; u32 pid = bpf_get_current_pid_tgid();
        31:   mov    %eax,-0x4(%rbp)
      ...
      -bash-4.4$ sudo bpftool p d xlated id 165
      int trace_connect_v6_return(struct pt_regs * ctx):
      ; int trace_connect_v6_return(struct pt_regs *ctx)
         0: (bf) r6 = r1
      ; int ret = PT_REGS_RC(ctx);
         1: (79) r7 = *(u64 *)(r6 +80)
      ; u32 pid = bpf_get_current_pid_tgid();
         2: (85) call bpf_get_current_pid_tgid#76208
      ; u32 pid = bpf_get_current_pid_tgid();
         3: (63) *(u32 *)(r10 -4) = r0
      ; skpp = bpf_map_lookup_elem((void *)bpf_pseudo_fd(1, 3), &pid);
         4: (18) r1 = map[id:298]
         6: (bf) r2 = r10

[4ast]

I think would have been cleaner to create pure c++ wrapper for btf_* where btf__new() would have been a constructor and btf__free() called in destructor. Another pure c++ wrapper for btf_ext__*(). Than this class would be called BCC_BTF, since it has this 'adjust()' workaround and private members at this line would be unique_ptr and unique_ptr<BTF_EXT>.
May be such c++ wrappers should be in github/libbpf/ repo.
Such refactoring can be done later.
Overall great stuff!

[yonghong-song]

Agree. I think some extensible C++ wrapper will be good for common use. Implementing in libbpf repo certainly is a great way for reusability. As you suggested, will try to refactor later.