[cmake] Also create lowercase extension WinSDK symlinks

Some projects rely on using libraries from the Windows SDK with their
original casing, just with a lowercase extension. E.g. the WinSock2 lib
is named WS2_32.Lib in the Windows SDK, and we would previously only
create a ws2_32.lib symlink for it (i.e. all lowercase). Also create a
WS2_32.lib symlink (i.e. original casing with lowercase extension) to
cover users of this casing. As a drive-by fix, only create these
symlinks when they differ from the original name to reduce the amount of
noise in the library symlinks directory.

llvm-svn: 343832