ARC: build: remove unneeded extra-y

Adding vmlinux.* to extra-y has no point because we expect they are
built on demand while building uImage.*

Add them to 'targets' is enough to include the corresponding .cmd file.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Signed-off-by: Vineet Gupta <vgupta@synopsys.com>