Initial support of TIZEN platform

Initial support of TIZEN platform
(part of commit 9379c66 from Unity-Technologies/bdwgc)

Issue #173 (bdwgc).

* include/private/gcconfig.h [TIZEN] (PLATFORM_TIZEN): New macro.
* include/private/gcconfig.h [I386 && LINUX && __ELF__
&& PLATFORM_TIZEN] (SEARCH_FOR_DATA_START, GC_NO_SIGSETJMP): Define.
* include/private/gcconfig.h [ARM32 && LINUX && __ELF__
&& PLATFORM_TIZEN] (SEARCH_FOR_DATA_START): Likewise.
* include/private/gcconfig.h [PLATFORM_TIZEN && !GETPAGESIZE]
(GETPAGESIZE): Define to sysconf(_SC_PAGESIZE).
* include/private/gcconfig.h [CAN_HANDLE_FORK] (CAN_CALL_ATFORK): Do
not define if PLATFORM_TIZEN.