perf callchain: Create an address space per thread

The unw_addr_space_t in libunwind represents an address space to be used
for stack unwinding.  It doesn't need to be create/destory everytime to
unwind callchain (as in get_entries) and can have a same lifetime as
thread (unless exec called).

So move the address space construction/destruction logic to the thread
lifetime handling functions.  This is a preparation to enable caching in
the unwind library.

Note that it saves unw_addr_space_t object using thread__set_priv().  It
seems currently only used by perf trace and perf kvm stat commands which
don't use callchain.

Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Acked-by: Jean Pihet <jean.pihet@linaro.org>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Cc: Arun Sharma <asharma@fb.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Jean Pihet <jean.pihet@linaro.org>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Namhyung Kim <namhyung.kim@lge.com>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/1412556363-26229-3-git-send-email-namhyung@kernel.org


[ Fixup unwind-libunwind.c missing CALLCHAIN_DWARF definition, added
  missing __maybe_unused on unused parameters in stubs at util/unwind.h ]
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>