Fuchsia simple support

This adds fairly standard support for Fuchsia in src/util. It's being used in downstream forks of Lavapipe and it's useful for gfxstream-vk. The idea is to incrementally merge these obvious changes to help reduce the patch load until someone has time to upstream the full driver.

These were authored by @castout, and reviewed by me. My rationale is I see experimental code in Mesa for drivers that aren't ready under the experimental flag (for example, nvk was experimental for a while, and so were other drivers before UAPI was merged). This is a similar case, at-least in my humble opinion.

Curious what other mesa-devs think.