In 'contrib/config-list.mk', clarify i686-symbolics-gnu to i686-gnu

Already in the first revision of 'contrib/config-list.mk', i686-symbolics-gnu
has been present, but it's not clear to me whether that was meant to be
Symbolics as in the manufacturer, <https://en.wikipedia.org/wiki/Symbolics>,
with GNU (that is, GNU/Hurd) kernel/operating system (user land), or Symbolics
kernel with GNU operating system (user land)?

I can't find any mention of "Symbolics" in the history of 'config.sub'
upstream.

Either way, GCC configures i686-symbolics-gnu exactly the same as i686-gnu:

    $ sed -n -e '/Using .* host machine hooks\.$/q' -e '/^Using the following target machine macro files:$/,$p' log/i686-gnu-make.out
    Using the following target machine macro files:
            [...]/gcc/config/vxworks-dummy.h
            [...]/gcc/config/i386/i386.h
            [...]/gcc/config/i386/unix.h
            [...]/gcc/config/i386/att.h
            [...]/gcc/config/elfos.h
            [...]/gcc/config/gnu-user.h
            [...]/gcc/config/glibc-stdint.h
            [...]/gcc/config/i386/gnu-user-common.h
            [...]/gcc/config/i386/gnu-user.h
            [...]/gcc/config/gnu.h
            [...]/gcc/config/i386/gnu.h
            [...]/gcc/config/initfini-array.h

..., so let's clarify i686-symbolics-gnu to i686-gnu.

	contrib/
	* config-list.mk (LIST): Clarify i686-symbolics-gnu to i686-gnu.