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/

rungdbserver

@@ -16,11 +16,11 @@ done
 shift "$(($OPTIND - 1))"
 executable="$1"
 set_common_vars "$arch" "$gem5"
-"${out_dir}/host/usr/bin/${arch}-linux-gdb" \
+"${host_dir}/usr/bin/${arch}-linux-gdb" \
   -q \
-  -ex "set sysroot ${out_dir}/staging" \
+  -ex "set sysroot ${buildroot_out_dir}/staging" \
   -ex 'target remote localhost:45455' \
   -ex 'tb main' \
   -ex 'c' \
-  "${out_dir}/build/${executable}" \
+  "${build_dir}/${executable}" \
 ;