Add support for FreeBSD

[kkebo]

I've added support for FreeBSD. I've only tested this PR on FreeBSD, but I believe it should also work on OpenBSD.

Motivation:

closes #372

Also, Swift on FreeBSD preview toolchain is now available.

Modifications:

• changed pthread-related code for *BSD
• changed Glibc.stdout and Glibc.stderr for *BSD because they are non-optional on *BSD

Result:

swift-log and packages depending on it will be usable on FreeBSD.

Checks:

I tested it with swift build and swift test. All tests have passed.

$ swift test
...
Test Suite 'All tests' passed at 2025-11-07 04:29:34.003                                                                                         
         Executed 0 tests, with 0 failures (0 unexpected) in 0.0 (0.0) seconds
...
✔ Test run with 52 tests in 7 suites passed after 0.309 seconds.

Environment:

$ uname -a
FreeBSD Coconut-rhinoceros-beetle 15.0-BETA4 FreeBSD 15.0-BETA4 releng/15.0-n280841-a7707f2a3bf4 GENERIC amd64
$ swift --version
Swift version 6.3-dev (LLVM bff1370bd79c983, Swift 57cf4ce563f700b)
Target: x86_64-unknown-freebsd14.3
Build config: +assertions

Swift toolchain is from https://forums.swift.org/t/swift-on-freebsd-preview/83064.