cmd/go: look for tool build ID before hashing entire file

Also fix the key used to store the ID.
    
    This is a significant speedup in cmd/go run time.
    
    Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/194257

From-SVN: r275559

gcc/go/gofrontend/MERGE

@@ -1,4 +1,4 @@
-5c3f52ffbae7a9bb59bce63cd2cffdd8af8f4a92
+68038b4fdf1456482af986cb05dcf3121bd43ffc
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.

libgo/go/cmd/go/internal/work/buildid.go

@@ -291,14 +291,19 @@ func (b *Builder) gccgoToolID(name, language string) (string, error) {
 				exe = lp
 			}
 		}
-		if _, err := os.Stat(exe); err != nil {
-			return "", fmt.Errorf("%s: can not find compiler %q: %v; output %q", name, exe, err, out)
+		id, err = buildid.ReadFile(exe)
+		if err != nil {
+			return "", err
+		}
+
+		// If we can't find a build ID, use a hash.
+		if id == "" {
+			id = b.fileHash(exe)
 		}
-		id = b.fileHash(exe)
 	}
 
 	b.id.Lock()
-	b.toolIDCache[name] = id
+	b.toolIDCache[key] = id
 	b.id.Unlock()
 
 	return id, nil