hurd: Add shared mig declarations

hurd: Add shared mig declarations

We are using -Werror=missing-declarations, and the _S.h files generated
by mig do not currently include a declaration for the server routine.
gnu-nat.c used to have its own external declarations, but better just
share them between gnu-nat.c and the _S.c files.

Fixes

exc_request_S.c:177:24: error: no previous declaration for ‘exc_server’ [-Werror=missing-declarations]
  177 | mig_external boolean_t exc_server

gdb/ChangeLog:

	* config/i386/i386gnu.mn [%_S.o %_U.o] (COMPILE.post): Add
	"-include gnu-nat-mig.h".
	* gnu-nat-mig.h: New file.
	* gnu-nat.c: Include "gnu-nat-mig.h".
	(exc_server, msg_reply_server, notify_server,
	process_reply_server): Remove declarations.