Cygwin: memory: declare extended memory API

Cygwin: memory: declare extended memory API

Windows 10 1803 introduced an extended memory API allowing
to specify memory regions allocations are to be taken off.

In preparation of using this API, define the struct
MEM_EXTENDED_PARAMETER and friends.  Declare and allow to
autoload the functions VirtualAlloc2 and NtMapViewOfSectionEx.

Introduce a wincap flag has_extended_mem_api.

Signed-off-by: Corinna Vinschen <corinna@vinschen.de>