arch/x86/assembly_entry: Align stack for car_stage_entry

At 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 and change the method for executing
car_stage_entry from jmp to call which should preserve proper
alignment regardless of a 32- or 64-bit build.

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: I8a15514f551a8e17e9fe77b8402fe0d2b106972e
Signed-off-by: Marshall Dawson <marshalldawson3rd@gmail.com>
Reviewed-on: https://review.coreboot.org/20528
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Aaron Durbin <adurbin@chromium.org>

src/arch/x86/assembly_entry.S

@@ -63,7 +63,8 @@ debug_spinloop:
 	jz	debug_spinloop
 #endif
 
-	jmp	car_stage_entry
+	andl	$0xfffffff0, %esp
+	call	car_stage_entry
 
 /* This is here for linking purposes. */
 .weak car_stage_entry