x86, asmlinkage: Make _*_start_kernel visible

Obviously these functions have to be visible, otherwise
the whole kernel could be optimized away.

Signed-off-by: Andi Kleen <ak@linux.intel.com>
Link: http://lkml.kernel.org/r/1375740170-7446-5-git-send-email-andi@firstfloor.org


Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>