arc: fix separate compilation of start.o

While testing "arc: make sure _start is in the beginning of .text
section" I haven't done proper clean-up of built binaries and so missed
another tiny bit that lead to the following error:
 --->8---
    LD      u-boot
 arc-linux-ld.bfd: cannot find arch/arc/lib/start.o
 Makefile:1107: recipe for target 'u-boot' failed
 make: *** [u-boot] Error 1
 --->8---

Fix is trivial: put "start.o" in "extra-y".

Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>