x86: qemu: Remove dead code

start.S does nothing and can be safely removed. Makefile is still being used
by the build system, so simply replace the rule in it. We use stub C-file
like it's done for other boards.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>