perf script: Assume native_arch for pipe mode

In pipe mode, session->header.env.arch is not populated until the events
are processed. Therefore, the following command crashes:

   perf record -o - | perf script

(gdb) bt

It fails when we try to compare env.arch against uts.machine:

        if (!strcmp(uts.machine, session->header.env.arch) ||
            (!strcmp(uts.machine, "x86_64") &&
             !strcmp(session->header.env.arch, "i386")))
                native_arch = true;

In pipe mode, it is tricky to find env.arch at this stage. To keep it
simple, let's just assume native_arch is always true for pipe mode.

Reported-by: David Carrillo Cisneros <davidca@fb.com>
Signed-off-by: Song Liu <songliubraving@fb.com>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: kernel-team@fb.com
Cc: stable@vger.kernel.org #v5.1+
Fixes: 3ab481a1

 ("perf script: Support insn output for normal samples")
Link: http://lkml.kernel.org/r/20190621014438.810342-1-songliubraving@fb.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>