x/build: overlayFiles for fix math on amd64

internal/build/build.go

@@ -111,6 +111,10 @@ const (
 	loadSyntax  = loadTypes | packages.NeedSyntax | packages.NeedTypesInfo
 )
 
+var overlayFiles = map[string]string{
+	"math/exp_amd64.go": "package math;",
+}
+
 func Do(args []string, conf *Config) {
 	flags, patterns, verbose := ParseArgs(args, buildFlags)
 	cfg := &packages.Config{
@@ -119,6 +123,13 @@ func Do(args []string, conf *Config) {
 		Fset:       token.NewFileSet(),
 	}
 
+	if len(overlayFiles) > 0 {
+		cfg.Overlay = make(map[string][]byte)
+		for file, src := range overlayFiles {
+			cfg.Overlay[filepath.Join(runtime.GOROOT(), "src", file)] = []byte(src)
+		}
+	}
+
 	llssa.Initialize(llssa.InitAll)
 	if verbose {
 		llssa.SetDebug(llssa.DbgFlagAll)