[POSIX] Reexport TOOLCHAINS when building.

 - This is very convenient when used with xcrun's indirection on OS X, it means
   you can do this:

     env TOOLCHAINS=swift Utilities/bootstrap

   and the build will automatically run against the latest toolchain (no PATH
   munging required).

   This depends a tiny bit on features from Xcode 7.3 beta, but once that GMs we
   should update the docs to make this the standard development-from-snapshot
   build instructions.

Author: ddunbar

Sources/POSIX/system.swift

@@ -49,7 +49,7 @@ func posix_spawnp(path: String, args: [String], environment: [String: String] =
     let argv = args.map{ $0.withCString(strdup) }
     defer { for arg in argv { free(arg) } }
 
-    for key in ["PATH", "SDKROOT", "HOME", "SWIFT_EXEC",
+    for key in ["PATH", "SDKROOT", "TOOLCHAINS", "HOME", "SWIFT_EXEC",
                 // FIXME these
                 "SPM_INSTALL_PATH", "SWIFT_BUILD_TOOL"] {
         environment[key] = POSIX.getenv(key)