www/reproxy: fix build on riscv64

Bump golang.org/x/sys dependency to a version new enough to
support riscv64-freebsd. While we are at it, remove useless
GH_TUPLE entries---the main distfile already has these vendored.

Sponsored by: Berliner Linux User Group e.V.
Approved by: portmgr (build fix blanket)
Differential Revision: https://reviews.freebsd.org/D39159