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>

# m68k specific configuration file

## depends on EXPERIMENTAL

##

## select ARCH_SUPPORTS_32

## select ARCH_DEFAULT_32

## select ARCH_DEFAULT_BE

## select ARCH_SUPPORT_CPU

##

## help The m68k architecture