Fix instances of '#if __LP64__'.

Triggers -Wundef, which is on in -Weverything.

Bug: http://b/31496165
Change-Id: Ib06107073f7dd1d584c19c222d0430da9d35630b
diff --git a/libc/upstream-openbsd/android/include/arith.h b/libc/upstream-openbsd/android/include/arith.h
index b262e4f..cb116d4 100644
--- a/libc/upstream-openbsd/android/include/arith.h
+++ b/libc/upstream-openbsd/android/include/arith.h
@@ -16,7 +16,7 @@
 
 #define IEEE_8087
 
-#if __LP64__
+#if defined(__LP64__)
 #define Long int
 #endif