[i386] Add data32 prefixes to all lgdt/lidt instructions

With a 16-bit operand, lgdt/lidt will load only a 24-bit base address,
ignoring the high-order bits.  This meant that we could fail to fully
restore the GDT across a call into gPXE, if the GDT happened to be
located above the 16MB mark.

Not all of our lgdt/lidt instructions require a data32 prefix (for
example, reloading the real-mode IDT can never require a 32-bit base
address), but by adding them everywhere we will hopefully not forget
the necessary ones in future.

src/arch/i386/prefix/libprefix.S

@@ -341,7 +341,7 @@ pm_call:
 	/* Switch CPU to protected mode and load up segment registers */
 	pushl	%eax
 	cli
-	lgdt	PM_CALL_VAR(gdt)(%bp)
+	data32 lgdt PM_CALL_VAR(gdt)(%bp)
 	movl	%cr0, %eax
 	orb	$CR0_PE, %al
 	movl	%eax, %cr0
@@ -377,7 +377,7 @@ pm_call:
 	popw	%es
 	popw	%fs
 	popw	%gs
-	lgdt	PM_CALL_VAR(pm_saved_gdt)(%bp)
+	data32 lgdt PM_CALL_VAR(pm_saved_gdt)(%bp)
 	popfl
 	movw	%bp, %sp
 	popw	%bp

src/arch/i386/transitions/librm.S

@@ -203,8 +203,8 @@ real_to_prot:
 
 	/* Switch to protected mode */
 	cli
-	data32 lgdt	gdtr
-	data32 lidt	idtr
+	data32 lgdt gdtr
+	data32 lidt idtr
 	movl	%cr0, %eax
 	orb	$CR0_PE, %al
 	movl	%eax, %cr0
@@ -316,7 +316,7 @@ p2r_jump_target:
 	movl	%edx, %esp
 
 	/* Reset IDTR to the real-mode defaults */
-	lidt	rm_idtr
+	data32 lidt rm_idtr
 
 	/* Return to real-mode address */
 	data32 ret
@@ -424,8 +424,8 @@ prot_call:
 1:	
 	/* Reload GDT and IDT, restore registers and flags and return */
 	movw	%sp, %bp
-	lgdt	(%bp)
-	lidt	8(%bp)
+	data32 lgdt (%bp)
+	data32 lidt 8(%bp)
 	addw	$20, %sp /* also skip %cs and %ss */
 	popw	%ds
 	popw	%es