Fix overflow errors in sbttous and sbttoms

Both of these functions would overflow for very large inputs. Add tests
for them. Also, add tests for the inverse functions, *stosbt, whose
overflow errors were fixed by 4c30b9ecd47.

PR: 263073
MFC after: 1 week
Sponsored by: Axcient
Reviewed by: imp
Differential Revision: https://reviews.freebsd.org/D34809