x86: Fix rom version build with CONFIG_X86_RESET_VECTOR

When building U-Boot with CONFIG_X86_RESET_VECTOR, the linking
process misses the resetvec.o and start16.o so it cannot generate
the rom version of U-Boot. The arch/x86/cpu/Makefile is updated to
pull them into the final linking process.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Acked-by: Simon Glass <sjg@chromium.org>