Fix instances of '#if __LP64__'.

Triggers -Wundef, which is on in -Weverything.

Bug: http://b/31496165
Change-Id: Ib06107073f7dd1d584c19c222d0430da9d35630b
diff --git a/libc/include/stdint.h b/libc/include/stdint.h
index 1cedca1..322a81c 100644
--- a/libc/include/stdint.h
+++ b/libc/include/stdint.h
@@ -39,7 +39,7 @@
 typedef unsigned short __uint16_t;
 typedef int __int32_t;
 typedef unsigned int __uint32_t;
-#if __LP64__
+#if defined(__LP64__)
 typedef long __int64_t;
 typedef unsigned long __uint64_t;
 #else
@@ -47,7 +47,7 @@
 typedef unsigned long long __uint64_t;
 #endif
 
-#if __LP64__
+#if defined(__LP64__)
 typedef long __intptr_t;
 typedef unsigned long __uintptr_t;
 #else