perf annotate browser: Add 'P' hotkey to dump annotation to file

perf annotate browser: Add 'P' hotkey to dump annotation to file

Just like we have in the histograms browser used as the main screen for
'perf top --tui' and 'perf report --tui', to print the current
annotation to a file with a named composed by the symbol name and the
".annotation" suffix.

Here is one example of pressing 'A' on 'perf top' to live annotate a
kernel function and then press 'P' to dump that annotation, the
resulting file:

  # cat _raw_spin_lock_irqsave.annotation
  _raw_spin_lock_irqsave() /proc/kcore
  Event: cycles:ppp

    7.14        nop
   21.43        push   %rbx
    7.14        pushfq
                pop    %rax
                nop
                mov    %rax,%rbx
                cli
                nop
                xor    %eax,%eax
                mov    $0x1,%edx
   64.29        lock   cmpxchg %edx,(%rdi)
                test   %eax,%eax
              ↓ jne    2b
                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-zzmnrwugb5vtk7bvg0rbx150@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>