Fast forward almost a decade since this question was originally posted and 64-bit is now the way to run .NET Function apps in isolated process.

If you do not run in 64-bit you get this warning in the Azure portal:

Availability risk alerts
Your .NET Isolated Function app is not running as a 64 bit process. This app is not optimized to avoid long cold start times. To avoid this issue please ensure that your app is set to run as a 64 bit process. This is documented at Guide for running C# Azure Functions in an isolated worker process