cmd/syncthing: Report real hashing performance, including weakhash

Instead of

    [I6KAH] 19:05:56 INFO: Single thread hash performance is 359 MB/s using minio/sha256-simd (354 MB/s using crypto/sha256).

it now says

    [I6KAH] 19:06:16 INFO: Single thread SHA256 performance is 359 MB/s using minio/sha256-simd (354 MB/s using crypto/sha256).
    [I6KAH] 19:06:17 INFO: Actual hashing performance is 299.01 MB/s

which is more informative. This is also the number it reports in usage
reporting.

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/3918

cmd/syncthing/main.go

@@ -613,6 +613,8 @@ func syncthingMain(runtimeOptions RuntimeOptions) {
 
 	sha256.SelectAlgo()
 	sha256.Report()
+	perf := cpuBench(3, 150*time.Millisecond)
+	l.Infof("Actual hashing performance is %.02f MB/s", perf)
 
 	// Emit the Starting event, now that we know who we are.