x86: use initial_code for i386

x86_64 jumps to whatever is written in "initial_code" symbol,
instead of a fixed address. Do it for i386 too. It will allow us
to integrate more of the smp boot code.

Signed-off-by: Glauber Costa <gcosta@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>