arm64: don't pull uaccess.h into *.S

Split asm-only parts of arm64 uaccess.h into a new header and use that
from *.S.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
diff --git a/arch/arm64/lib/clear_user.S b/arch/arm64/lib/clear_user.S
index add4a13..e88fb99 100644
--- a/arch/arm64/lib/clear_user.S
+++ b/arch/arm64/lib/clear_user.S
@@ -17,7 +17,7 @@
  */
 #include <linux/linkage.h>
 
-#include <linux/uaccess.h>
+#include <asm/asm-uaccess.h>
 
 	.text