Fix for PR c++/40808

gcc/cp/ChangeLog:

	PR c++/40808
	* mangle.c (write_template_args): Allow mangling of empty template
	argument list. Updated function comments.

gcc/testsuite/ChangeLog:

	PR c++/40808
	* g++.dg/abi/mangle34.C: New test

From-SVN: r153517