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.

Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/216077

gcc/go/gofrontend/MERGE

-7d3081ce69dda123d77e35e8b9d282e40e9465e2
+10a8dbfc9945c672d59af8edb9790e2019cdeb27
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.

libgo/go/internal/cpu/cpu_riscv64.go

@@ -3,5 +3,3 @@
 // license that can be found in the LICENSE file.
 
 package cpu
-
-const CacheLinePadSize = 32