build: move FreeBSD to experimental

The FreeBSD platform arguably doesn't have enough dedicated maintainers to be considered a Tier 2 platform. However I think it does match the requirements for Experimental:

These are often working to be promoted to Tier 2 but are not quite ready.
There is at least one individual actively providing maintenance and the team
is striving to broaden quality and reliability of support.

AFAICT Johan (@jbergstroem) has been singlehandedly maintaining support. We get a fair number of FreeBSD specific issues, most recently with V8 6.0 (which I don't think officially supports FreeBSD).

This is intended as a conversation starter, I have no strong feelings about this. If there are actually a bunch of people in @nodejs/platform-freebsd who have the time to fix issues (or if I just didn't notice...) then this can be closed.

To clarify, making FreeBSD experimental doesn't mean that we'll stop testing our xLinux builds on the platform, it just means that we won't necessarily be blocked on FreeBSD specific issues on (for example) V8 upgrades.

We could also change it back any time, so this wouldn't have to be a permanent thing.

Refs: nodejs#14384 Refs: nodejs/build#723 Refs: https://github.com/nodejs/node-v8/issues/14

Checklist

• documentation is changed or added
• commit message follows commit guidelines

Affected core subsystem(s)

build