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="Harold Grovesteen"

reporter_url="http://sourceware.org/ml/crossgcc/2009-11/msg00052.html"

reporter_comment=""