fix doxygen for IceBuildDefs.h

fix doxygen for IceBuildDefs.h

This is an attempt to make IceBuildDefs more understandable
in the Doxygen produced output, as well as when the various
functions are later referenced from other parts of the subzero docs.

If you look at the doxygen for both the file and the IceBuildDefs namespace, I think it's definitely much
clearer.

I'm still learning Doxygen and always see new things and more that I could have done but Rome was not built in a day.

In my browser:
file:///home/rkotler/nacl_dir/native_client/toolchain_build/src/subzero/docs/html/namespaceIce_1_1BuildDefs.html

and

file:///home/rkotler/nacl_dir/native_client/toolchain_build/src/subzero/docs/html/IceBuildDefs_8h.html

BUG=
R=stichnot@chromium.org

Review URL: https://codereview.chromium.org/1519113003 .

Patch from Reed Kotler <rkotlerimgtec@gmail.com>.