libc/newlib: add option to optimise for size

Add an option to build with -Os instead of the default -O.

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

reporter_name="Yann E. MORIN"

reporter_url="http://ymorin.is-a-geek.org/dokuwiki/projects/crosstool"

reporter_comment=""