RTMemSafer: Split generic from ring-3 specific page based implementation. Adjusted the ring-3 implementation to use RTMemPage when SUPR3PageAllocEx isn't available, and to separate the real data from the heap metadata to make finding the data just a little bit more difficult.

IPRT: Make RTMemSafer use the SUPR3 page allocation if available to allocate locked down memory. Change API to make allocation strategy tweakable to allow for pageable allocations where the support library is not available (build tools like bldRTSignTool) while still making sure the memory is non-pageable for sensitive data