[rt][nfc] Rewrite #ifndef as #if defined().

GitOrigin-RevId: f58a1f65e74686b7a59ae2fa849ce4c70d520e89
diff --git a/tests/backtrace.cpp b/tests/backtrace.cpp
index 7a3427e..a4eb8eb 100644
--- a/tests/backtrace.cpp
+++ b/tests/backtrace.cpp
@@ -46,7 +46,7 @@
 }
 
 TEST_F(BacktraceGuardedPoolAllocatorDeathTest, UseAfterFree) {
-#ifdef __linux__ && __ARM_ARCH == 7
+#if defined(__linux__) && __ARM_ARCH == 7
   // Incomplete backtrace on Armv7 Linux
   GTEST_SKIP();
 #endif