perf env: Also consider env->arch == NULL as local operation

perf env: Also consider env->arch == NULL as local operation

We'll set a new machine field based on env->arch, which for live mode,
like with 'perf top' means we need to use uname() to figure the name of
the arch, fix perf_env__arch() to consider both (env == NULL) and
(env->arch == NULL) as local operation.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: David S. Miller <davem@davemloft.net>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Leo Yan <leo.yan@linaro.org>
Cc: Mathieu Poirier <mathieu.poirier@linaro.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Wang Nan <wangnan0@huawei.com>
Cc: stable@vger.kernel.org # 4.19
Link: https://lkml.kernel.org/n/tip-vcz4ufzdon7cwy8dm2ua53xk@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>