Move all build and run output to out/$arch/

Most of it was present inside buildroot/output.* and the rest scattered
on top level.

This came about for the n-th time when we were reviewing QEMU trace file
locations.

On one hand, it would be cool to have per arch traces.

This made buildroot/output.${arch}~/ a natural choice.

But on the other, those traces have nothing to do with Buildroot,
and could potentially interfere with Buildroot build files.

It also feels nicer to have buildroot/ pristine source code only,
and keep all output under a single directory out/

rungdb-user

@@ -30,8 +30,8 @@ else
   brk=''
 fi
 set_common_vars "$arch" "$gem5"
-executable="${out_dir}/build/${executable_rel}"
-readelf="${out_dir}/host/usr/bin/${arch}-linux-readelf"
+executable="${build_dir}/${executable_rel}"
+readelf="${host_dir}/usr/bin/${arch}-linux-readelf"
 addr="$("$readelf" -h "$executable" | awk '/Entry/{ print $NF }' )"
 ex="-ex \"add-symbol-file $executable $addr\""
 # -L or else lx-symbols throws for arm: