support new Windows enclave feature

Windows 10 TH2 ("1511") (xref #1825) has added system calls involving "enclaves".

Xref WinAPI routines like InitializeEnclave:
https://msdn.microsoft.com/en-us/library/windows/desktop/mt592869(v=vs.85).aspx

Enclave: freed with VirtualFree.
Enclaves require Intel Software Guard Extensions (SGX).
Basically a protected area within address space.

Will require DR support and likely will complicate DR operation as its code cache (and DR lib?) may have to be split to have part inside the enclave.