stdlib: define typealias CLongDouble for FreeBSD on i386, x86_64

This fixes a failure compiling the stdlib where the importer cannot find (i.e.,
refuses to import) functions _stdlib_remainderl and _stdlib_squareRootl.

Author: mhjacobson

stdlib/public/core/CTypes.swift

@@ -101,6 +101,11 @@ public typealias CLongDouble = Double
 #endif
 #elseif os(OpenBSD)
 public typealias CLongDouble = Float80
+#elseif os(FreeBSD)
+#if arch(x86_64) || arch(i386)
+public typealias CLongDouble = Float80
+#endif
+// TODO: Fill in definitions for additional architectures as needed.
 #endif
 
 // FIXME: Is it actually UTF-32 on Darwin?