Looks like, not surprisingly, it's TARGET_OS_TV.
From TargetConditionals.h for tvOS 9.0 Simulator:
/
* gcc based compiler used on Mac OS X
*/
#if defined(__GNUC__) && ( defined(__APPLE_CPP__) || defined(__APPLE_CC__) || defined(__MACOS_CLASSIC__) )
#define TARGET_OS_MAC 1
#define TARGET_OS_WIN32 0
#define TARGET_OS_UNIX 0
#define TARGET_OS_IPHONE 1
#define TARGET_OS_IOS 0
#define TARGET_OS_WATCH 0
#define TARGET_OS_TV 1
#define TARGET_OS_SIMULATOR 1
#define TARGET_OS_EMBEDDED 0
#define TARGET_IPHONE_SIMULATOR TARGET_OS_SIMULATOR / deprecated */
#define TARGET_OS_NANO TARGET_OS_WATCH / deprecated */
...