[NaCl] Enable building of shared libraries under glibc.

The opt level build-nacl.sh script will now build in glibc
configuration as well as newlib and in this case it will
produce shared objects rather than static libraries for
the core cocos2dx library as well as the box2d library.

R=binji@chromium.org

Review URL: https://codereview.chromium.org/15001038