devel/etcd31: fix build on riscv64

Bump golang.org/x/sys and github.com/coreos/bbolt to versions that
support risc64-freebsd.

Sponsored by: Berliner Linux User Group e.V.
Approved by: olgeni (maintainer)
Approved by: eduardo (mentor)
Differential Revision: https://reviews.freebsd.org/D38863