Please bump golang.org/x/sys dependency to enable a build on riscv64-freebsd

[clausecker]

The version of golang.org/x/sys you depend on is too outdated to allow a build on riscv64-freebsd. Please bump the dependency so a build is possible.

[aleksraiden]

As I think, it will be fixed after merge my PR: #969

[clausecker]

The issue has already been resolved, golang.org/x/sys@v0.6.0 is sufficient and you're already way past that.