Remove process_stratum_target::hostio_last_error abstraction

Now that the WinCE port is gone, all ports map host I/O errors from
errno, so this abstraction is no longer necessary.

Basically undoes:
  https://sourceware.org/pipermail/gdb-patches/2008-January/055511.html
  https://sourceware.org/pipermail/gdb-patches/attachments/20080131/f44e7012/attachment.bin

gdbserver/ChangeLog:

	* Makefile.in (SFILES): Remove hostio-errno.cc.
	* configure: Regenerate.
	* configure.ac (GDBSERVER_DEPFILES): No longer add
	$srv_hostio_err_objs.
	* configure.srv (srv_hostio_err_objs): Delete.
	* hostio-errno.cc: Delete.
	* hostio.cc (hostio_error): Inline hostio_last_error_from_errno
	here.
	* hostio.h (hostio_last_error_from_errno): Delete.
	* target.cc (process_stratum_target::hostio_last_error): Delete.
	* target.h (class process_stratum_target) <hostio_last_error>:
	Delete.