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

arch/x86/kernel/head_32.S

@@ -455,7 +455,10 @@ is386:	movl $2,%ecx		# set MP
 	jmp initialize_secondary # all other CPUs call initialize_secondary
 1:
 #endif /* CONFIG_SMP */
-	jmp i386_start_kernel
+	jmp *(initial_code)
+.align 4
+ENTRY(initial_code)
+	.long i386_start_kernel
 
 /*
  * We depend on ET to be correct. This checks for 287/387.