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

@@ -42,8 +42,8 @@ else
   port=1234
 fi
 set_common_vars "$arch" "$gem5"
-gdb="${out_dir}/host/usr/bin/${arch}-linux-gdb ${before}"
-cd "${out_dir}/build/linux-custom/"
+gdb="${host_dir}/usr/bin/${arch}-linux-gdb ${before}"
+cd "${build_dir}/linux-custom/"
 if "$kgdb"; then
     cmd="\
 ${gdb} \