cmd/syncthing: Add selectable sha256 package (fixes #3613, fixes #3614)

This adds autodetection of the fastest hashing library on startup, thus
handling the performance regression. It also adds an environment
variable to control the selection, STHASHING=standard (Go standard
library version, avoids SIGILL crash when the minio library has bugs on
odd CPUs), STHASHING=minio (to force using the minio version) or unset
for the default autodetection.

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/3617

cmd/syncthing/monitor.go

@@ -178,6 +178,22 @@ func copyStderr(stderr io.Reader, dst io.Writer) {
 		if panicFd == nil {
 			dst.Write([]byte(line))
 
+			if strings.Contains(line, "SIGILL") {
+				l.Warnln(`
+*******************************************************************************
+* Crash due to illegal instruction detected. This is most likely due to a CPU *
+* incompatibility with the high performance hashing package. Switching to the *
+* standard hashing package instead. Please report this issue at:              *
+*                                                                             *
+*   https://github.com/syncthing/syncthing/issues                             *
+*                                                                             *
+* Include the details of your CPU.                                            *
+*******************************************************************************
+`)
+				os.Setenv("STHASHING", "standard")
+				return
+			}
+
 			if strings.HasPrefix(line, "panic:") || strings.HasPrefix(line, "fatal error:") {
 				panicFd, err = os.Create(timestampedLoc(locPanicLog))
 				if err != nil {