Load dynamic cpu lib on windows

On linux, we link the CPU library in to the Go app and fall back to it
when no GPU match is found. On windows we do not link in the CPU library
so that we can better control our dependencies for the CLI.  This fixes
the logic so we correctly fallback to the dynamic CPU library
on windows.

llm/ext_server_windows.go

@@ -1,8 +1,6 @@
 package llm
 
 import (
-	"fmt"
-
 	"github.com/jmorganca/ollama/api"
 )
 
@@ -10,6 +8,5 @@ func newDefaultExtServer(model string, adapters, projectors []string, numLayers
 	// On windows we always load the llama.cpp libraries dynamically to avoid startup DLL dependencies
 	// This ensures we can update the PATH at runtime to get everything loaded
 
-	// Should not happen
-	return nil, fmt.Errorf("no default impl on windows - all dynamic")
+	return newDynamicShimExtServer(AvailableShims["cpu"], model, adapters, projectors, numLayers, opts)
 }