remove unused CTOR_LISTS_DEFINED_EXTERNALLY macro

The last target to use this was i386-interix, so since that is gone we
don't need this anymore.

libgcc/ChangeLog:

2016-07-06  Trevor Saunders  <tbsaunde+gcc@tbsaunde.org>

	* libgcc2.c (SYMBOL__MAIN): Remove checks for
	CTOR_LISTS_DEFINED_EXTERNALLY.

From-SVN: r238067