eval.c:evaluate_subexp_standard: Factor out function call handling

While working on the no-debug-info debugging improvements, I found
evaluate_subexp_standard's function call code unnecessarily long and
hard to navigate and debug.  The use of goto doesn't help either.

This commit tries to improve things by factoring out the
function-call-related code to separate helper functions.

gdb/ChangeLog:
2017-09-05  Pedro Alves  <palves@redhat.com>

	* eval.c (eval_call, evaluate_funcall): New functions, factored
	out from ...
	(evaluate_subexp_standard): ... this.