Use only get_virtual_memory_page_size()

[jodyhagins]

virtual_memory_page_size has been deprecated, but it was being used internally extensively. Prior to this change, static use was impossible if PAGESIZE or PAGE_SIZE weren't set.

The deprecation has been taken one step forward in that virtual_memory_page_size is no longer available by default. This is a breaking change, but the previous behavior can be enabled with FOONATHAN_MEMORY_ALLOW_VIRTUAL_MEMORY_PAGE_SIZE.