`available: arch != "arm32"` appears to be unnecessary

[WardBrian]

This appeared first in core v.0.16.1, and is also found in many of the transitive dependencies (e.g base), but it appears to be incorrect.

Using an alpine docker image which was emulating arm/v7, I was able to remove these bounds from both the v.0.16.2 release and the v.0.17.1 release and still successfully install core and its dependencies. Furthermore, I could successfully build and test a decently large program which uses core

I understand that Jane Street doesn't want to test these platforms, but could this available: clause be relaxed so that they can still be installed and tested by others?