x86: fix comments in vmlinux_64.lds

for bzImage, the vmlinux_64.lds still have s32 bit code, and startup_32
should be 0. fix the comment.
Signed-off-by: NYinghai Lu <yinghai.lu@sun.com>
Signed-off-by: NIngo Molnar <mingo@elte.hu>

arch/x86/boot/compressed/vmlinux_64.lds

@@ -3,7 +3,7 @@ OUTPUT_ARCH(i386:x86-64)
 ENTRY(startup_64)
 SECTIONS
 {
-	/* Be careful parts of head_64.S assume startup_64 is at
+	/* Be careful parts of head_64.S assume startup_32 is at
 	 * address 0.
 	 */
 	. = 0;