Add multiple CPU variants for Intel Mac

This also refines the build process for the ext_server build.

llm/payload_darwin_arm64.go

@@ -0,0 +1,8 @@
+package llm
+
+import (
+	"embed"
+)
+
+//go:embed llama.cpp/ggml-metal.metal llama.cpp/build/darwin/arm64/*/lib/*.dylib*
+var libEmbed embed.FS