[gdbserver] return ENOSYS if readlink not supported.

[gdbserver] return ENOSYS if readlink not supported.

This reproduces on the GDBserver side what GDB does when readlink is
not supported.

gdb/gdbserver/ChangeLog:

        * configure.ac: Add readlink to AC_CHECK_FUNCS list.
        * configure, config.in: Regenerate.
        * hostio.c: Provide an alternate implementation if HAVE_READLINK
        is not defined.