perf top: Show backtrace when handling a SIGSEGV on --stdio mode

perf top: Show backtrace when handling a SIGSEGV on --stdio mode

It was just freezing instead of informing about the SEGV, fix it and
also print a backtrace, just like in the TUI mode and in 'perf trace'.

Tested by provoking a NULL deref when pressing 'z':

     0.31%  libc-2.20.so     [.] malloc_consolidate
     0.31%  ld-2.20.so       [.] _dl_relocate_object
     0.28%  cc1              [.] ht_lookup
     0.28%  cc1              [.] ira_init_register_move_cost
  perf: Segmentation fault
  Obtained 7 stack frames.
  perf(dump_stack+0x32) [0x4d69f2]
  perf(sighandler_dump_stack+0x29) [0x4d6a89]
  /lib64/libc.so.6(+0x34960) [0x7f5064333960]
  perf() [0x438790]
  /lib64/libpthread.so.0(+0x752a) [0x7f50663dd52a]
  /lib64/libc.so.6(clone+0x6d) [0x7f50643ff22d]
  #

Reported-by: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Borislav Petkov <bp@suse.de>
Cc: David Ahern <dsahern@gmail.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lkml.kernel.org/n/tip-pewrpzqd29rgmhu2wkk7fhww@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>