i#1569 AArch64: Add missing "flags" argument in call to SYS_unlinkat.

This old bug explains why 97abbd4 broke client.drx-test on AArch64.

Review-URL: https://codereview.appspot.com/309590043

core/unix/os.c

@@ -4004,7 +4004,7 @@ os_delete_file(const char *name)
 #ifdef SYS_unlink
     return (dynamorio_syscall(SYS_unlink, 1, name) == 0);
 #else
-    return (dynamorio_syscall(SYS_unlinkat, 2, AT_FDCWD, name) == 0);
+    return (dynamorio_syscall(SYS_unlinkat, 3, AT_FDCWD, name, 0) == 0);
 #endif
 }