Add syncfs(2).

GMM calls this system call directly at the moment. That's silly.

Bug: http://b/36405699
Test: ran tests
Change-Id: I1e14c0e5ce0bc2aa888d884845ac30dc20f13cd5
diff --git a/libc/libc.arm64.map b/libc/libc.arm64.map
index 50e4390..ce6f12d 100644
--- a/libc/libc.arm64.map
+++ b/libc/libc.arm64.map
@@ -1240,9 +1240,10 @@
 
 LIBC_P {
   global:
-    __freading; # future
-    __fwriting; # future
-    getlogin_r; # future
+    __freading; # introduced=P
+    __fwriting; # introduced=P
+    getlogin_r; # introduced=P
+    syncfs; # introduced=P
 } LIBC_O;
 
 LIBC_PRIVATE {