fixed the pthread compilation errors by following Sangjin Han's advice,

see also Han's PR:  https://github.com/apple/swift/pull/3886/files#diff-99fd89ad44c4466de8ad6b8aab9c207d

Author: kstaring

stdlib/private/SwiftPrivatePthreadExtras/PthreadBarriers.swift

@@ -16,8 +16,6 @@ import Darwin
 import Glibc
 #endif
 
-// FIXME: On FreeBSD, code in this file doesn't compile
-#if !os(FreeBSD)
 //
 // Implement pthread barriers.
 //
@@ -45,8 +43,13 @@ public var _stdlib_PTHREAD_BARRIER_SERIAL_THREAD: CInt {
 }
 
 public struct _stdlib_pthread_barrier_t {
+#if CYGWIN || os(FreeBSD)
+  var mutex: UnsafeMutablePointer<pthread_mutex_t?>? = nil
+  var cond: UnsafeMutablePointer<pthread_cond_t?>? = nil
+#else
   var mutex: UnsafeMutablePointer<pthread_mutex_t>? = nil
   var cond: UnsafeMutablePointer<pthread_cond_t>? = nil
+#endif
 
   /// The number of threads to synchronize.
   var count: CUnsignedInt = 0
@@ -131,4 +134,3 @@ public func _stdlib_pthread_barrier_wait(
     return _stdlib_PTHREAD_BARRIER_SERIAL_THREAD
   }
 }
-#endif

stdlib/private/SwiftPrivatePthreadExtras/SwiftPrivatePthreadExtras.swift

@@ -21,8 +21,6 @@ import Darwin
 import Glibc
 #endif
 
-// FIXME: On FreeBSD, code in this file doesn't compile
-#if !os(FreeBSD)
 /// An abstract base class to encapsulate the context necessary to invoke
 /// a block from pthread_create.
 internal class PthreadBlockContext {
@@ -61,9 +59,15 @@ internal func invokeBlockContext(
   return context.run()
 }
 
+#if CYGWIN || os(FreeBSD)
+public typealias _stdlib_pthread_attr_t = UnsafePointer<pthread_attr_t?>
+#else
+public typealias _stdlib_pthread_attr_t = UnsafePointer<pthread_attr_t>
+#endif
+
 /// Block-based wrapper for `pthread_create`.
 public func _stdlib_pthread_create_block<Argument, Result>(
-  _ attr: UnsafePointer<pthread_attr_t>?,
+  _ attr: _stdlib_pthread_attr_t?,
   _ start_routine: @escaping (Argument) -> Result,
   _ arg: Argument
 ) -> (CInt, pthread_t?) {
@@ -142,4 +146,3 @@ public class _stdlib_Barrier {
     }
   }
 }
-#endif