[pcbios] Save/restore %es in INT 15,e820

Our INT 15,e820 code was setting %es=%ss (as part of the "look ahead
in the memory map" logic), but failing to restore %es afterwards.
This is a serious bug, but wasn't affecting many platforms because
almost all callers seem to set %es=%ss anyway.

src/arch/i386/firmware/pcbios/e820mangler.S

 
 	/* Peek ahead to see if there are any further nonempty regions */
 	pushal
+	pushw	%es
 	subw	$20, %sp
 	movl	$0xe820, %eax
 	movl	$SMAP, %edx
 	movw	%sp, %di
 	call	get_nonempty_e820
 	addr32 leal 20(%esp), %esp /* avoid changing flags */
+	popw	%es
 	popal
 	jnc	99f /* There are further nonempty regions */