Fix build issues on BSDs in pkg/cpu (#7116)

Also add a cross compile script to test always cross
compilation for some well known platforms and architectures
, we support out of box compilation of these platforms even
if we don't make an official release build.

This script is to avoid regressions in this area when we
add platform dependent code.

cmd/erasure.go

@@ -93,10 +93,12 @@ func (e *Erasure) DecodeDataAndParityBlocks(ctx context.Context, data [][]byte)
 	return nil
 }
 
+// ShardSize - returns actual shared size from erasure blockSize.
 func (e *Erasure) ShardSize() int64 {
 	return ceilFrac(e.blockSize, int64(e.dataBlocks))
 }
 
+// ShardFileSize - returns final erasure size from original size.
 func (e *Erasure) ShardFileSize(totalLength int64) int64 {
 	if totalLength == 0 {
 		return 0
@@ -107,6 +109,7 @@ func (e *Erasure) ShardFileSize(totalLength int64) int64 {
 	return numShards*e.ShardSize() + lastShardSize
 }
 
+// ShardFileTillOffset - returns the effectiv eoffset where erasure reading begins.
 func (e *Erasure) ShardFileTillOffset(startOffset, length, totalLength int64) int64 {
 	shardSize := e.ShardSize()
 	shardFileSize := e.ShardFileSize(totalLength)