stdlib: define RTLD_DEFAULT for FreeBSD

It's defined the same as on Darwin and OpenBSD.

Author: mhjacobson

stdlib/private/StdlibUnittest/SymbolLookup.swift

@@ -23,7 +23,7 @@
 #error("Unsupported platform")
 #endif
 
-#if canImport(Darwin) || os(OpenBSD)
+#if canImport(Darwin) || os(OpenBSD) || os(FreeBSD)
   let RTLD_DEFAULT = UnsafeMutableRawPointer(bitPattern: -2)
 #elseif os(Linux) || os(WASI)
   let RTLD_DEFAULT = UnsafeMutableRawPointer(bitPattern: 0)