libc/newlib: Add extra config for newlib.

So users can input config like --enable-newlib-register-fini.

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

reporter_name="Kevin Cernekee"

reporter_url="http://sourceware.org/ml/crossgcc/2011-09/msg00037.html"

reporter_comment="MIPS SDE-compatible toolchain"