Add partial support for FreeBSD

This change will allow `swift build` to succeed. However, SwiftPM support is still incomplete and does not yet allow adding `.freebsd` to `.when(platforms:)`, so `swift test` cannot be executed.

Author: kkebo

Sources/AsyncAlgorithms/Locking.swift

@@ -29,7 +29,11 @@ internal struct Lock {
   #if canImport(Darwin)
   typealias Primitive = os_unfair_lock
   #elseif canImport(Glibc) || canImport(Musl) || canImport(Bionic) || canImport(wasi_pthread)
+  #if os(FreeBSD) || os(OpenBSD)
+  typealias Primitive = pthread_mutex_t?
+  #else
   typealias Primitive = pthread_mutex_t
+  #endif
   #elseif canImport(WinSDK)
   typealias Primitive = SRWLOCK
   #else