perf env: Introduce read_cpu_topology_map() method

Out of the code to write the cpu topology map in the perf.data file
header.

Now if one needs the CPU topology map for the running machine, one needs
to call perf_env__read_cpu_topology_map(perf_env) and the info will be
stored in perf_env.cpu.

For now we're using a global perf_env variable, that will have its
contents freed after we run a builtin.

v2: Check perf_env__read_cpu_topology_map() return in
    write_cpu_topology() (Kan Liang)

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: Kan Liang <kan.liang@intel.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Stephane Eranian <eranian@google.com>
Cc: Wang Nan <wangnan0@huawei.com>
Link: http://lkml.kernel.org/r/1441828225-667-5-git-send-email-acme@kernel.org


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>