compiler: truncate output files before writing

compiler.go

@@ -3180,7 +3180,7 @@ func (c *Compiler) EmitText(path string) error {
 // Write the data to the file specified by path.
 func (c *Compiler) writeFile(data []byte, path string) error {
 	// Write output to file
-	f, err := os.OpenFile(path, os.O_RDWR|os.O_CREATE, 0666)
+	f, err := os.OpenFile(path, os.O_RDWR|os.O_CREATE|os.O_TRUNC, 0666)
 	if err != nil {
 		return err
 	}

main.go

@@ -114,7 +114,7 @@ func Build(pkgName, outpath, target string, printIR, dumpSSA bool) error {
 				return err
 			}
 			defer inf.Close()
-			outf, err := os.OpenFile(outpath, os.O_RDWR|os.O_CREATE, 0777)
+			outf, err := os.OpenFile(outpath, os.O_RDWR|os.O_CREATE|os.O_TRUNC, 0777)
 			if err != nil {
 				return err
 			}