Use #if TARGET_OS_IOS instead of #ifdef TARGET_OS_IOS.

runtime/bin/platform_macos.cc

Index: runtime/bin/platform_macos.cc
diff --git a/runtime/bin/platform_macos.cc b/runtime/bin/platform_macos.cc
index e970d6e957299f54054c54329ba341da5e7ba1d8..54b48d6a071392933d182398390e9c6526c08d1f 100644
--- a/runtime/bin/platform_macos.cc
+++ b/runtime/bin/platform_macos.cc
@@ -12,9 +12,9 @@
 #include "bin/file.h"
 #include "bin/platform.h"
 
-#if !defined(TARGET_OS_IOS)
+#if !TARGET_OS_IOS
 #include <crt_externs.h>  // NOLINT
-#endif  // !defined(TARGET_OS_IOS)
+#endif  // !TARGET_OS_IOS
 #include <signal.h>  // NOLINT
 #include <string.h>  // NOLINT
 #include <unistd.h>  // NOLINT
@@ -68,7 +68,7 @@ bool Platform::LocalHostname(char *buffer, intptr_t buffer_length) {
 
 
 char** Platform::Environment(intptr_t* count) {
-#if defined(TARGET_OS_IOS)
+#if TARGET_OS_IOS
   // TODO(iposva): On Mac (desktop), _NSGetEnviron() is used to access the
   // environ from shared libraries or bundles. This is present in crt_externs.h
   // which is unavailable on iOS. On iOS, everything is statically linked for