Cygwin: winlean.h: remove most of the extended memory API

Cygwin: winlean.h: remove most of the extended memory API

This was added as a temporary measure in commit e18f7f99 because it
wasn't yet in the mingw-w64 headers.  With one exception, it is now in
the current release of the headers (version 8.0.0), so we don't need
it in winlean.h.

The exception is that VirtualAlloc2 is declared conditionally in
<w32api/memoryapi.h>, but the compilation of Cygwin requires it to
always be declared, even though it will only be executed on systems
that support it.  So retain the declaration in winlean.h.  And add
"WINAPI" to the declaration, as in memoryapi.h.

Add a check that version >= 8 of the mingw-w64 headers is intalled.

Also revert commit 3d136011, which was a related temporary workaround.