linux-user: disable unicore32 linux-user build

In order to cleanup linux-user, we need support for most relatively
modern syscalls. unicore32 lacks support for syscalls like
epoll_pwait, preventing cleaning up the CONFIG_EPOLL mess.

This patch can be reverted when unicore32 starts either supporting
the syscalls as defined in mainline kernel, or the oldabi interface
gains support for syscalls supported since at kernel 2.6.19 / glibc 2.6

Cc: MPRC <zhangheng@mprc.pku.edu.cn>
Cc: Xuetao Guan <gxt@mprc.pku.edu.cn>
Signed-off-by: Riku Voipio <riku.voipio@linaro.org>