Compress better on amd64 (#12974)

Since S2 has amd64 assembly, it now operates at a reasonable 
speed to use by default.

Here are some examples of stream compression speed, 16 cores:
```
nyc-taxi-data-10M.csv	s2	1	3325605752	-> 1095998837	312ms	10139.07MB/s		67.04% reduction
nyc-taxi-data-10M.csv	s2	2	3325605752	-> 917905514	428ms	7393.74MB/s		72.40%

github-june-2days-2019.json	s2	1	6273951764	-> 1043196283	391ms	15301.99 MB/s		83.37%
github-june-2days-2019.json	s2	2	6273951764	-> 955924506	519ms	11510.81MB/s		84.76%

github-ranks-backup.bin	s2	1	1862623243	-> 623911363	146ms	12133MB/s		66.50%
github-ranks-backup.bin	s2	2	1862623243	-> 563752759	230ms	7705.26MB/s		69.73%
```

We keep non-assembly platforms on the faster, but less efficient mode.

cmd/object-api-utils.go

@@ -884,6 +884,16 @@ func CleanMinioInternalMetadataKeys(metadata map[string]string) map[string]strin
 	return newMeta
 }
 
+// compressOpts are the options for writing compressed data.
+var compressOpts []s2.WriterOption
+
+func init() {
+	if runtime.GOARCH == "amd64" {
+		// On amd64 we have assembly and can use stronger compression.
+		compressOpts = append(compressOpts, s2.WriterBetterCompression())
+	}
+}
+
 // newS2CompressReader will read data from r, compress it and return the compressed data as a Reader.
 // Use Close to ensure resources are released on incomplete streams.
 //
@@ -894,7 +904,7 @@ func newS2CompressReader(r io.Reader, on int64) io.ReadCloser {
 	pr, pw := io.Pipe()
 	// Copy input to compressor
 	go func() {
-		comp := s2.NewWriter(pw)
+		comp := s2.NewWriter(pw, compressOpts...)
 		cn, err := io.Copy(comp, r)
 		if err != nil {
 			comp.Close()