x86/boot: Clear direction and interrupt flags in startup_64

startup_32 already clears these flags on entry, do it in startup_64 as
well for consistency.

The direction flag in particular is not specified to be cleared in the
boot protocol documentation, and we currently call into C code
(paging_prepare) without explicitly clearing it.
Signed-off-by: NArvind Sankar <nivedita@alum.mit.edu>
Link: https://lore.kernel.org/r/20200202171353.3736319-5-nivedita@alum.mit.eduSigned-off-by: NArd Biesheuvel <ardb@kernel.org>

arch/x86/boot/compressed/head_64.S

@@ -264,6 +264,9 @@ SYM_CODE_START(startup_64)
 	 * and command line.
 	 */
 
+	cld
+	cli
+
 	/* Setup data segments. */
 	xorl	%eax, %eax
 	movl	%eax, %ds