libc/newlib: remove hard-coded "-O"

With hard-coded "-O", users can not customize CFLAGS_FOR_TARGET
by CT_TARGET_CFLAGS. If "-O" is needed, users can input it in
CT_TARGET_CFLAGS. By default, "-Os" is enabled.

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

 reporter_name="Dragan Marinkovic <marinkovic.dragan@gmail.com>"

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