Add support for FreeBSD

[grynspan]

Add support for FreeBSD:

• Audit our platform-specific code and add branches for FreeBSD (mostly, but not entirely, the same as Linux)
  • Add platform-specific branches for FreeBSD. #685
  • Additional FreeBSD fixes #865
  • Fix more things on FreeBSD #867
• Set up team workstations or VMs
• Set up CI jobs

See also: swiftlang/swift#68723

[michael-yuji]

PR created for platform-specific code #865

[grynspan]

Much appreciated!

[grynspan]

Tracked internally as rdar://135874298.