cc/gcc: Update core_prefix_dir to prefix.

core_prefix_dir is not defined. It should be prefix.

Signed-off-by: Zhenqiang Chen <zhenqiang.chen@linaro.org>

 reporter_name="Ioannis E. Venetis"

 reporter_url="http://sourceware.org/ml/crossgcc/2011-05/msg00005.html"

 reporter_comment=""