perf env: Add routine to read the env->cpuid from the running machine

perf env: Add routine to read the env->cpuid from the running machine

In 'perf top' we use that cpuid when initializing the per arch
annotation init routines (e.g. x86__annotate_init()) and in that case
(live mode, 'perf top') we need to obtain it from the running machine,
not from a perf.data file header.

Provide a means to do that. Will be used by 'perf top' in a followup
patch.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Link: https://lkml.kernel.org/n/tip-h2wb3sx7u7znx6lqfezrh7ca@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>