[FreeBSD]: Optional observation pthread primitives

The pthread APIs on FreeBSD do not include nullability APIs so the
pthread mutex APIs are imported as pointers to optional values.
Other platforms include nullability APIs and import the pthread APIs as
pointers to the mutex. Splitting the locking primitive type based on the
OS.

Fixes: rdar://150880976

Author: etcwilde

stdlib/public/Observation/Sources/Observation/Locking.swift

@@ -34,7 +34,11 @@ internal struct Lock {
   #if canImport(Darwin)
   typealias Primitive = os_unfair_lock
   #elseif canImport(Glibc) || canImport(Musl) || canImport(Bionic)
+  #if os(FreeBSD)
+  typealias Primitive = pthread_mutex_t?
+  #else
   typealias Primitive = pthread_mutex_t
+  #endif
   #elseif canImport(WinSDK)
   typealias Primitive = SRWLOCK
   #elseif arch(wasm32)