all: fix staticcheck warnings

This is a loose collection of small fixes flagged by staticcheck:

  - dead code
  - regexp expressions not using backticks (`foobar` / "foobar")
  - redundant types of slice and map initializers
  - misc other fixes

Not all of these seem very useful to me, but in particular dead code is
nice to fix. I've fixed them all just so that if there are problems,
they aren't hidden in the noise of less useful issues.

builder/ar.go

@@ -27,7 +27,7 @@ func makeArchive(archivePath string, objs []string) error {
 	arwriter := ar.NewWriter(arfile)
 	err = arwriter.WriteGlobalHeader()
 	if err != nil {
-		return &os.PathError{"write ar header", archivePath, err}
+		return &os.PathError{Op: "write ar header", Path: archivePath, Err: err}
 	}
 
 	// Open all object files and read the symbols for the symbol table.

builder/uf2.go

@@ -141,11 +141,13 @@ func split(input []byte, limit int) [][]byte {
 	var block []byte
 	output := make([][]byte, 0, len(input)/limit+1)
 	for len(input) >= limit {
+		// add all blocks
 		block, input = input[:limit], input[limit:]
 		output = append(output, block)
 	}
 	if len(input) > 0 {
-		output = append(output, input[:len(input)])
+		// add remaining block (that isn't full sized)
+		output = append(output, input)
 	}
 	return output
 }