smm: Push a null stack base pointer

When generating a backtrace we need an indicator when we have hit the
beginning of the stack. The i386 ABI states that %ebp points to the next
stack frame. NULL can be used to indicate the end of the stack.

We could add a NULL return pointer at %ebp+4, but I decided to omit it
since a NULL stack pointer can be used as an indicator that there is no
return pointer.

BUG=b:80539294
TEST=built and tested on grunt

Change-Id: I8a48114d31a5c716335d264fa4fe4da41dc5bf11
Signed-off-by: Raul E Rangel <rrangel@chromium.org>
Reviewed-on: https://review.coreboot.org/27226
Reviewed-by: Martin Roth <martinroth@google.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/cpu/x86/smm/smm_stub.S

@@ -135,6 +135,9 @@ smm_trampoline32:
 	movl	stack_top, %edx
 	subl	%eax, %edx /* global_stack_top - offset = stack_top */
 	mov	%edx, %esp
+
+	/* Create stack frame by pushing a NULL stack base pointer */
+	pushl	$0x0
 	mov	%esp, %ebp
 
 	/* Allocate locals (fxsave) */