Sane default bitness for 32-bit on 64-bit hosts

[th0ma7]

Due to lack of proper bitness detection when cross-compiling a 32-bit target on 64-bit host this leads into inability to properly compile wheels that depends on this (such as using b2 requiring address-model=32)

This refers to the following discussion: arvidn/libtorrent#6792 (comment)

[benfogle]

I need to add that in, certainly. I just need to make sure that that value isn't used internally for anything important, especially from C code. At first glance, it appears that it's only used to set "really big" upper limits in a few places.