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>

# Compute Alpha-specific values

CT_DoArchTupleValues () {

    # The architecture part of the tuple:

    CT_TARGET_ARCH="${CT_ARCH}${CT_ARCH_ALPHA_VARIANT}"

}