Initial support of WinRT

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

Issue #173 (bdwgc).

* include/private/gc_priv.h [MSWIN32] (GET_TIME): Use GetTickCount64
if MSWINRT_FLAVOR (instead of GetTickCount).
* include/private/gcconfig.h [_MSC_VER && _M_IX86>=300
|| _WIN32 && !__CYGWIN__ && !SYMBIAN && _M_ARM] (ARM32): Define macro.
* include/private/gcconfig.h [(_MSC_VER && _M_IX86>=300
|| _WIN32 && !__CYGWIN__ && !SYMBIAN) && !_XBOX_ONE
&& WINAPI_FAMILY==WINAPI_FAMILY_APP] (MSWINRT_FLAVOR): Likewise.
* include/private/gcconfig.h [MSWINRT_FLAVOR] (NO_GETENV): Likewise.
* include/private/gcconfig.h [X86_64 && MSWIN32] (DATAEND): Do not
define if already defined.
* misc.c [MSWIN32] (GC_win32_MessageBoxA): Do not define if
MSWINRT_FLAVOR.
* misc.c [MSWIN32] (GC_init): Call
InitializeCriticalSectionAndSpinCount() directly if MSWINRT_FLAVOR
(instead of using GetProcAddress).
* misc.c [MSWIN32 && MSWINRT_FLAVOR]: Include windows.storage.h.
* misc.c [MSWIN32 && MSWINRT_FLAVOR] (RoGetActivationFactory): Declare.
* misc.c [MSWIN32 && MSWINRT_FLAVOR] (getWinRTLogPath): New static
function.
* misc.c [MSWIN32 && MSWINRT_FLAVOR] (GC_CreateLogFile): Use
getWinRTLogPath().
* os_dep.c [GWW_VDB && MSWINRT_FLAVOR] (detect_GetWriteWatch): Use
memInfo.AllocationBase to get hK32.
* os_dep.c [USE_WINALLOC && !MSWIN_XBOX1] (GC_win32_get_mem,
GC_win32_free_heap): Do not use GlobalAlloc and GlobalFree if
MSWINRT_FLAVOR.