Fix instances of '#if __LP64__'.

Triggers -Wundef, which is on in -Weverything.

Bug: http://b/31496165
Change-Id: Ib06107073f7dd1d584c19c222d0430da9d35630b
diff --git a/libc/include/link.h b/libc/include/link.h
index f40e7e4..92ecceb 100644
--- a/libc/include/link.h
+++ b/libc/include/link.h
@@ -34,7 +34,7 @@
 
 __BEGIN_DECLS
 
-#if __LP64__
+#if defined(__LP64__)
 #define ElfW(type) Elf64_ ## type
 #else
 #define ElfW(type) Elf32_ ## type