superenv: Allow ENV.m32 (for 32bit builds)

Superenv normally filters out "-m32" flag, preventing 32bit builds.
Some software, however, still only work in 32bit mode.

If ENV.m32 is called, superenv does not filter out the "-m32" flag.
Also note, superenv, does not explicitly add the -m32 flag and
expects the build system of the software to know when and where to
provide this flag.