Fix register_dynamic_libraries on Windows 10

Fix register_dynamic_libraries on Windows 10

Issue #219 (bdwgc).

In the past (Windows XP, Windows 7, and older Windows 10), VirtualQuery
always returned PAGE_READWRITE for the data section.
In the April 2018 Update of Windows 10, it seems that PAGE_WRITECOPY
is returned sometimes (thus causing GC_register_dynamic_libraries not
to call GC_cond_add_roots for the section).

* dyn_load.c [MSWIN32 || MSWINCE || CYGWIN32]
(GC_register_dynamic_libraries): Call GC_cond_add_roots()
also for PAGE_EXECUTE_WRITECOPY and PAGE_WRITECOPY pages.