Initial support of Orbis target

Initial support of Orbis target
(part of commit 9379c66 from Unity-Technologies/bdwgc)

Issue #173 (bdwgc).

* alloc.c [!MACOS && !MSWINCE && !__CC_ARM]: Do not include
sys/types.h if SN_TARGET_ORBIS.
* dyn_load.c [!MACOS && !_WIN32_WCE && !__CC_ARM]: Likewise.
* include/gc_config_macros.h [SN_TARGET_ORBIS] (GC_THREADS): Define.
* include/gc_config_macros.h [__FreeBSD__] (GC_FREEBSD_THREADS): Do
not define if SN_TARGET_ORBIS.
* include/private/gcconfig.h [__FreeBSD__ || __DragonFly__
|| __FreeBSD_kernel__] (FREEBSD): Likewise.
* include/private/gcconfig.h [!LINUX && !NETBSD && !FREEBSD
&& !OPENBSD && !DARWIN && !_WIN32 && !__CEGCC__ && !NN_PLATFORM_CTR
!NN_BUILD_TARGET_PLATFORM_NX && !SYMBIAN] (NOSYS): Likewise.
* include/private/gc_locks.h [THREADS && GC_PTHREADS
&& !GC_WIN32_THREADS]: Do not include gc_atomic_ops.h if
SN_TARGET_ORBIS.
* include/private/gc_locks.h [SN_TARGET_ORBIS && GC_PTHREADS]
(USE_PTHREAD_LOCKS): Define.
* include/private/gc_priv.h [SN_TARGET_ORBIS] (CPP_LOG_HBLKSIZE):
Define to 16 (instead of 12).
* include/private/gcconfig.h [SN_TARGET_ORBIS && (__amd64__
|| __x86_64__)] (X86_64): Define.
* include/private/gcconfig.h [X86_64] (CACHE_LINE_SIZE): Do not define
if already defined.
* include/private/gcconfig.h [X86_64 && SN_TARGET_ORBIS] (DATASTART,
DATAEND, STACKBOTTOM): Define.
* include/private/gcconfig.h [X86_64 && SN_TARGET_ORBIS]: Include
pthread.h.
* include/private/gcconfig.h [X86_64 && SN_TARGET_ORBIS]
(ps4_get_stack_bottom): Declare.
* include/private/gcconfig.h [USE_MUNMAP && !MUNMAP_THRESHOLD
&& SN_TARGET_ORBIS] (MUNMAP_THRESHOLD): Define to 2.
* include/private/gcconfig.h [SN_TARGET_ORBIS] (THREADS): Define.
* pthread_support.c: Do not include sys/mman.h, sys/time.h,
sys/types.h, fcntl.h if SN_TARGET_ORBIS.
* pthread_support.c (GC_pthread_join, GC_pthread_detach,
GC_pthread_create): Do not define if SN_TARGET_ORBIS.