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. I am looking at you, smlnj.

With this change, If ENV.m32 is called, superenv does not filter out the "-m32" flag as it would usually.
Also note, superenv, does **no**t explicitly add the -m32 flag, nor any "-arch" flags but
expects the build system of the software to know when and where to provide this flag.

I would need this behavior to ready my upcomping changes of smlnj. Superenv really shines there, because I don't need to patch smlnj in hundred ways to build with Xcode but it just works (except that smlnj is only 32bit).