[prefix] Align INT 15,88 temporary decompression area to a page boundary

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/arch/x86/prefix/libprefix.S

 	movw	%ax, %di
 	addl	$0x400, %edi
 	subl	$_textdata_memsz_kb, %edi
+	andw	$~0x03, %di
 	shll	$10, %edi
 	/* Sanity check: if we have ended up below 1MB, use 1MB */
 	cmpl	$0x100000, %edi