perf thread: Generalize function to copy from thread addr space from intel-bts code

perf thread: Generalize function to copy from thread addr space from intel-bts code

Add a utility function to fetch executable code. Convert one
user over to it. There are more places doing that, but they
do significantly different actions, so they are not
easy to fit into a single library function.

Committer changes:

. No need to cast around, make 'buf' be a void pointer.

. Rename it to thread__memcpy() to reflect the fact it is about copying
  a chunk of memory from a thread, i.e. from its address space.

. No need to have it in a separate object file, move it to thread.[ch]

. Check the return of map__load(), the original code didn't do it, but
  since we're moving this around, check that as well.

Signed-off-by: Andi Kleen <ak@linux.intel.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Link: https://lkml.kernel.org/r/20190305144758.12397-2-andi@firstfloor.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>