Use #if TARGET_OS_IOS instead of #ifdef TARGET_OS_IOS.

runtime/platform/globals.h

Index: runtime/platform/globals.h
diff --git a/runtime/platform/globals.h b/runtime/platform/globals.h
index 0d493100639328a25337fa200339be9289fefad9..db73d62365750ac733b4c1e4604747a5ceaaa669 100644
--- a/runtime/platform/globals.h
+++ b/runtime/platform/globals.h
@@ -95,7 +95,7 @@
 // the value defined in TargetConditionals.h
 #define TARGET_OS_MACOS 1
 #if TARGET_OS_IPHONE
-#define TARGET_OS_IOS
+#define TARGET_OS_IOS 1
 #endif
 
 #elif defined(_WIN32)
@@ -279,17 +279,17 @@ typedef simd128_value_t fpu_register_t;
 #elif defined(TARGET_ARCH_X64)
   // No simulator used.
 #elif defined(TARGET_ARCH_ARM)
-#if !defined(HOST_ARCH_ARM) || defined(TARGET_OS_IOS)
+#if !defined(HOST_ARCH_ARM) || TARGET_OS_IOS
 #define USING_SIMULATOR 1
 #endif
 
 #elif defined(TARGET_ARCH_ARM64)
-#if !defined(HOST_ARCH_ARM64) || defined(TARGET_OS_IOS)
+#if !defined(HOST_ARCH_ARM64) || TARGET_OS_IOS
 #define USING_SIMULATOR 1
 #endif
 
 #elif defined(TARGET_ARCH_MIPS)
-#if !defined(HOST_ARCH_MIPS) || defined(TARGET_OS_IOS)
+#if !defined(HOST_ARCH_MIPS) || TARGET_OS_IOS
 #define USING_SIMULATOR 1
 #endif