Build failure against riscv64

```
github.com/jacobsa/crypto/cmac
golang.org/x/sys/cpu
# github.com/jacobsa/crypto/cmac
vendor/github.com/jacobsa/crypto/cmac/hash.go:97:3: undefined: xorBlock
```

See: http://autobuild.buildroot.net/results/caa60874781c4077273884eb37281cc9e02ef9ac/build-end.log