x86/lapic/secondary.S: Align stack for _secondary_start

At a process _start, the stack is expected to be aligned to a
16-byte boundary.  Upon entry to any function the stack frame
must have the end of any arguments also aligned.  In other words
the value of %esp+4 or %rsp+8 is always a multiple of 16 (1).

Align the stack down inside _secondary_start and preserve proper
alignment for the call to secondary_cpu_init.

Although 4-byte alignment is the minimum requirement for i386,
some AMD platforms use SSE instructions which expect 16-byte.

1) http://wiki.osdev.org/System_V_ABI
   See "Initial Stack and Register State" and "The Stack Frame"
   in the supplements.

BUG=chrome-os-partner:62841664

Change-Id: I72b7a474013e5caf67aedfabeb8d8d2553499b73
Signed-off-by: Marshall Dawson <marshalldawson3rd@gmail.com>
Reviewed-on: https://review.coreboot.org/20537
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Aaron Durbin <adurbin@chromium.org>

src/cpu/x86/lapic/secondary.S

@@ -69,6 +69,8 @@ __ap_protected_start:
 	/* Set the stack pointer, and flag that we are done */
 	xorl	%eax, %eax
 	movl	secondary_stack, %esp
+	andl	$0xfffffff0, %esp
+	sub	$12, %esp /* maintain 16-byte alignment for the call below */
 	movl	secondary_cpu_index, %edi
 	pushl	%edi
 	movl	%eax, secondary_stack