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="Remy Bohmer"

 reporter_url="linux@bohmer.net"

 reporter_comment="Example for building a toolchain for m68000 bare metal targets"