Darwin, libgfortran : Do not use environ directly from the library.

On macOS / Darwin, the environ variable can be used directly in the
code of an executable, but cannot be used in the code of a shared
library (i.e. libgfortran.dylib), in this case.

In such cases, the  function _NSGetEnviron should be called to get
the address of 'environ'.

libgfortran/ChangeLog:

	* intrinsics/execute_command_line.c (environ): Use
	_NSGetEnviron to get the environment pointer on Darwin.