perf annotate: Add function header to --stdio2

perf annotate: Add function header to --stdio2

  # perf annotate --stdio2 _raw_spin_lock_irqsave
  _raw_spin_lock_irqsave() /lib/modules/4.16.0-rc4/build/vmlinux
  Event: anon group { cycles, instructions }

    0.00   3.17      → callq  __fentry__
    0.00   7.94        push   %rbx
    7.69  36.51      → callq  __page_file_index
                       mov    %rax,%rbx
    7.69   3.17      → callq  *ffffffff82225cd0
                       xor    %eax,%eax
                       mov    $0x1,%edx
   80.77  49.21        lock   cmpxchg %edx,(%rdi)
                       test   %eax,%eax
                     ↓ jne    2b
    3.85   0.00        mov    %rbx,%rax
                       pop    %rbx
                     ← retq
                 2b:   mov    %eax,%esi
                     → callq  queued_spin_lock_slowpath
                       mov    %rbx,%rax
                       pop    %rbx
                     ← retq
  #

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jin Yao <yao.jin@linux.intel.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Wang Nan <wangnan0@huawei.com>
Link: https://lkml.kernel.org/n/tip-i86yfyzl8m194ioxgj1jo32f@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>