Cygwin: Don't dump non-writable image regions

After this, we will end up dumping memory regions where:

- state is MEM_COMMIT (i.e. is not MEM_RESERVE or MEM_FREE), and
-- type is MEM_PRIVATE and protection allows reads (i.e. not a guardpage), or
-- type is MEM_IMAGE and protection allows writes

Making this decision based on the current protection isn't 100% correct,
because it may have been changed using VirtualProtect().  But we don't
know how to determine if a region is shareable.

(As a practical matter, anything which gets us the stack (MEM_PRIVATE)
and .data/.bss (RW MEM_IMAGE) is going to be enough for 99% of cases)

winsup/utils/dumper.cc

@@ -292,6 +292,12 @@ dumper::collect_memory_sections ()
       int skip_region_p = 0;
       const char *disposition = "dumped";
 
+      if ((mbi.Type & MEM_IMAGE) && !(mbi.Protect & (PAGE_EXECUTE_READWRITE | PAGE_READWRITE)))
+	{
+	  skip_region_p = 1;
+	  disposition = "skipped due to non-writeable MEM_IMAGE";
+	}
+
       if (mbi.Protect & PAGE_NOACCESS)
 	{
 	  skip_region_p = 1;