gdb: Add an overloaded ui_out::text accepting a const std::string &

gdb: Add an overloaded ui_out::text accepting a const std::string &

gdb/ChangeLog:

	* ui-out.h (class ui_out): Add ui_out::text accepting a constant
	reference to a std::string.  Fix all callers using
	std::string::c_str.
	* ui-out.c (ui_out::text): Ditto.