builddeb: generate multi-arch friendly linux-libc-dev package

Debian-based distributions place libc header files in a machine
specific directory (/usr/include/<libc-machine>) instead of
/usr/include/asm to support installation of the linux-libc-dev
package from multiple architectures. Move headers installed by
"make headers_install" accordingly using Debian's tuple from
dpkg-architecture (stored in debian/arch).

Signed-off-by: Cedric Hombourger <Cedric_Hombourger@mentor.com>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>