Enable compilation for Cygwin with MPROTECT_VDB

Enable compilation for Cygwin with MPROTECT_VDB

Note: It just compiles but not working.

* include/private/gc_priv.h [GC_WIN32_THREADS && MPROTECT_VDB]
(GC_set_write_fault_handler): Do not declare if CYGWIN32.
* os_dep.c [MPROTECT_VDB && !DARWIN && !USE_WINALLOC]: Do not include
sys/syscall.h if CYGWIN32.
* os_dep.c [MPROTECT_VDB && CYGWIN32] (CODE_OK): Define (to true).
* os_dep.c [MPROTECT_VDB && GC_WIN32_THREADS]
(GC_set_write_fault_handler): Do not define if CYGWIN32.
* win32_threads.c [MPROTECT_VDB] (GC_register_my_thread_inner): Do not
call GC_gww_dirty_init() and GC_set_write_fault_handler() if CYGWIN32.