[Ada] Fix binding of ghost units with finalizer

[Ada] Fix binding of ghost units with finalizer

Linking of an enabled ghost unit which requires a finalizer lead to an
error, as the name generated by the binder for calling the finalizer was
not the same as the name chosen by the compiler. Now fixed.

2019-07-23  Yannick Moy  <moy@adacore.com>

gcc/ada/

	* exp_ch7.adb (Create_Finalizer): Force finalizer not to be
	Ghost enabled.
	* exp_dbug.adb (Get_External_Name): Explain special case of
	Ghost finalizer.

gcc/testsuite/

	* gnat.dg/ghost6.adb, gnat.dg/ghost6_pkg.ads: New testcase.

From-SVN: r273720