Bump default datasize on arm64 to 768M to prepare for building clang 7.

ok deraadt@, visa@

src/etc/etc.arm64/login.conf

@@ -1,4 +1,4 @@
-# $OpenBSD: login.conf,v 1.2 2017/02/06 18:11:33 sthen Exp $
+# $OpenBSD: login.conf,v 1.3 2019/01/24 13:14:10 kettenis Exp $
 
 #
 # Sample login.conf file.  See login.conf(5) for details.
@@ -41,8 +41,8 @@ auth-ftp-defaults:auth-ftp=passwd:
 default:\
 	:path=/usr/bin /bin /usr/sbin /sbin /usr/X11R6/bin /usr/local/bin /usr/local/sbin:\
 	:umask=022:\
-	:datasize-max=512M:\
-	:datasize-cur=512M:\
+	:datasize-max=768M:\
+	:datasize-cur=768M:\
 	:maxproc-max=256:\
 	:maxproc-cur=128:\
 	:openfiles-max=1024:\
@@ -71,7 +71,7 @@ daemon:\
 # Staff have fewer restrictions and can login even when nologins are set.
 #
 staff:\
-	:datasize-cur=512M:\
+	:datasize-cur=768M:\
 	:datasize-max=infinity:\
 	:maxproc-max=512:\
 	:maxproc-cur=128:\