Add spaceship operator to cp-name-parser.y

While debugging gdb, I saw this:

During symbol reading: unexpected demangled name 'operator<=><std::chrono::_V2::system_clock, std::chrono::duration<long int>, std::chrono::duration<long int> >'

This happens because cp-name-parser.y does not handle the spaceship
operator.  This patch implements this.

Approved-By: John Baldwin <jhb@FreeBSD.org>