Bump golang.org/x/sys dependency

[clausecker]

While packaging this project for FreeBSD (see PR 257233), I noticed that the golang.org/x/sys dependency requested by the project does not build on FreeBSD. I have included a patch to bump this dependency to v0.0.0-20210630005230-0f9fa26af87c, so the project builds for now. But for future versions, especially with the vendoring you perform, it would be a lot easier if you could just bump the dependency on your part.

[clausecker]

For the port to build on riscv64 FreeBSD, a bump to v0.5.0 is now necessary.