cpu: fix wrong cache line size of riscv64

This CL follows CL526659 as cache line should be 64 Bytes

Change-Id: Id3a49dbfbfad6aeb439eb5e585fd2d6228a3d3ed
Reviewed-on: https://go-review.googlesource.com/c/sys/+/526935
Reviewed-by: Heschi Kreinick <[email protected]>
Reviewed-by: Tobias Klauser <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Ian Lance Taylor <[email protected]>

Author: mengzhuo

Diff for: cpu/cpu_riscv64.go

@@ -7,6 +7,6 @@
 
 package cpu
 
-const cacheLineSize = 32
+const cacheLineSize = 64
 
 func initOptions() {}