internal/cpu: don't define CacheLinePadSize for riscv64
internal/cpu: don't define CacheLinePadSize for riscv64

In libgo CacheLinePadSize is defined by the generated file cpugen.go.

Keep cpu_riscv64.go around, even though it is now empty, so that
we will pick up changes to it in future merges.

Change-Id: Ia2a393a320327dffa88bd3c4f7a6571b36a1adad
Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/216077
Reviewed-by: Cherry Zhang <d62e63aa42ce272d7b6a5055d97e942b33a34679@google.com>

golang/gofrontend,golang/gofrontend,golang/gofrontend,golang/gofrontend