libcc1: share GDB plugin code

The two GDB plugins in libcc1 share a fair amount of code.  This was
done by copy-and-paste, though in reality the underlying code is
nearly identical.

libcc1

	* libcp1.cc (struct libcp1): Derive from base_gdb_plugin.  Remove
	shared code.
	(class libcp1_connection): Remove.
	(rpc): Remove.
	(libcp1_set_verbose, libcp1_compile): Update.
	(cp_call_binding_oracle, cp_call_symbol_address)
	(cp_call_enter_scope, cp_call_leave_scope): Update.
	* libcc1.cc (struct libcc1): Derive from base_gdb_plugin.  Remove
	shared code.
	(class libcc1_connection): Remove.
	(c_call_binding_oracle, c_call_symbol_address): Update.
	(rpc): Remove.
	(libcc1_set_verbose, libcc1_compile): Update.
	* gdbctx.hh: New file.