hurd: Avoid more libc.so PLTs

	* sysdeps/hurd/include/hurd/signal.h (_hurd_raise_signal): Add hidden
	prototype.
	* hurd/hurd-raise.c (_hurd_raise_signal): Add hidden def.
	* hurd/Makefile ($(inlines:%=$(objpfx)%.c): Define
	_HEADER_H_HIDDEN_DEF macro.
	* sysdeps/hurd/include/hurd/fd.h (_hurd_fd_error,
	_hurd_fd_error_signal): Add hidden prototype.
	[_HURD_FD_H_HIDDEN_DEF] (_hurd_fd_error, _hurd_fd_error_signal): Add
	hidden def.