Fix instances of '#if __LP64__'.

Triggers -Wundef, which is on in -Weverything.

Bug: http://b/31496165
Change-Id: Ib06107073f7dd1d584c19c222d0430da9d35630b
diff --git a/tests/sys_ptrace_test.cpp b/tests/sys_ptrace_test.cpp
index 9071acf..bdd6a89 100644
--- a/tests/sys_ptrace_test.cpp
+++ b/tests/sys_ptrace_test.cpp
@@ -183,7 +183,7 @@
   run_watchpoint_test_impl<uint8_t>(cpu);
   run_watchpoint_test_impl<uint16_t>(cpu);
   run_watchpoint_test_impl<uint32_t>(cpu);
-#if __LP64__
+#if defined(__LP64__)
   run_watchpoint_test_impl<uint64_t>(cpu);
 #endif
 }