[Patch, libada] Fix PR64349 - make darwin use the correct interface to access _environ.

[Eric Botcazou]

> 	PR ada/64349
> 	* env.c (__gnat_environ): Use the _NSGetEnviron() interface to access
> 	the environment for Darwin.

OK if Tristan has no objections to the patch:
  https://gcc.gnu.org/ml/gcc-patches/2014-12/msg01753.html

-- 
Eric Botcazou