Refactor handle_qxfer_libraries and friends to use std::string
Using std::string in handle_qxfer_libraries and friends allow to
simplify the code. We don't have to manually free the buffer, and we
don't have to pre-compute the required space.
gdb/gdbserver/ChangeLog:
* server.c (accumulate_file_name_length): Remove.
(emit_dll_description): Adjust to std::string change.
(handle_qxfer_libraries): Use std::string to hold document.