Bump golang.org/x/sys in go.mod

The ancient version of `golang.org/x/sys` referenced in `go.mod` seems to not build for `darwin/amd64` with current Go versions. Error is `//go:linkname must refer to declared function or variable`. It would be appreciated if the deps could be bumped to fix this.

Begin with the golang.org/x/sys entry in go.mod and check which newer version addresses the reported //go:linkname error on darwin/amd64. Verify the dependency update by building for darwin/amd64 with a current Go version; done means the build completes without that error.

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