Don't configure gdb for vxworks target

Don't configure gdb for vxworks target

VxWorks support was removed in 2004.

commit e84ecc995d6a5e4e9114d3cea61717b8a573afb6
Author: Andrew Cagney <cagney@redhat.com>
Date:   Sat Nov 13 23:10:02 2004 +0000

    2004-11-13  Andrew Cagney  <cagney@gnu.org>

        * configure.tgt: Delete i[34567]86-*-vxworks*, m68*-netx-*,
        m68*-*-vxworks*, mips*-*-vxworks*, powerpc-*-vxworks*, and
        sparc-*-vxworks*.
        * NEWS: Mention that vxworks was deleted.

This patch adds *-*-vxworks* in a list of targets GDB doesn't
support.

gdb:

2016-10-20  Yao Qi  <yao.qi@linaro.org>

	* configure.tgt: Don't configure if target is *-*-vxworks*.