kernel.bbclass: pass LD also in savedefconfig

kernel.bbclass: pass LD also in savedefconfig

* similar to:
  https://git.openembedded.org/openembedded-core/commit/?id=66c1f4b0abd3483759f9e4141a80f982eaf8ccfd
  https://git.openembedded.org/openembedded-core/commit/?id=36fe2efc91fec5a52374a2dc4814a69076a2d28e
  but for savedefconfig task
* otherwise with ld-is-gold it will fail with:
  gold linker is not supported as it is not capable of linking the kernel proper.

Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>