[WIP] FreeBSD support
I'm confused by some of the test fails - does a clean PR fail at the moment (due to a system update?) or has this PR got some unintended side effect?
@rhuanjl It seems like MacOS builds currently fail both on azure and cirrus (Even without changes) See #7007
That seems to be the case, I haven't looked into why MacOS failed yet, and style failed because I touched PAL.
With these changes (there are a couple of things needing better checks or further investigation) my build hits a wall with system libicu, which is a lot newer than Linux system version or the one we embed.