Use internal real-mode stack from non-returnable prefixes

src/arch/i386/core/stack16.S

@@ -0,0 +1,13 @@
+	.arch i386
+
+/****************************************************************************
+ * Internal stack
+ ****************************************************************************
+ */
+	.section ".stack16"
+	.align 8
+	.globl _stack16
+_stack16:
+	.space 4096
+	.globl _estack16
+_estack16:

src/arch/i386/prefix/dskprefix.S

@@ -350,6 +350,10 @@ msg1end:
 start_runtime:
 	call	install
 
+	/* Set up real-mode stack */
+	movw	%bx, %ss
+	movw	$_estack16, %sp
+
 	/* Jump to .text16 segment */
 	pushw	%ax
 	pushw	$1f

src/arch/i386/prefix/romprefix.S

@@ -139,6 +139,10 @@ exec_vector:
 
 	call	install
 
+	/* Set up real-mode stack */
+	movw	%bx, %ss
+	movw	$_estack16, %sp
+
 	/* Jump to .text16 segment */
 	pushw	%ax
 	pushw	$1f