perf annotate: Use perf_env when obtaining the arch name

perf annotate: Use perf_env when obtaining the arch name

Paving the way to reuse these routines in other areas, like when
generating errno tables.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Hendrik Brueckner <brueckner@linux.vnet.ibm.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Michael Petlan <mpetlan@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Thomas Richter <tmricht@linux.vnet.ibm.com>
Cc: Wang Nan <wangnan0@huawei.com>
Link: https://lkml.kernel.org/n/tip-rh1qv051vb8gfdcswskrn53h@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>