gdb: Use vector::emplace_back

gdb: Use vector::emplace_back

Now that we require C++11, we can use vector::emplace_back to
construct elements in place instead of constructing and then copying.

gdb/ChangeLog:
2016-11-09  Pedro Alves  <palves@redhat.com>

	* main.c (struct cmdarg): Add constructor.
	(captured_main_1): Use vector::emplace_back.
	* tracepoint.c (collection_list::add_memrange): Likewise.