security/cfssl: fix build on riscv64

Bump golang.org/x/sys dependency to a version new enough to
support riscv64-freebsd. This issue is already fixed up stream
and he patches can be taken out with the next update.

Sponsored by: Berliner Linux User Group e.V.
Approved by: portmgr (build fix blanket)
Differential Revision: https://reviews.freebsd.org/D39103