cmd/go: for gccgo, 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 when using an
unreleased toolchain. For example, the TestGoBuildTestOnly cmd/go test
goes from 15 seconds to 1 second.

Change-Id: Ibfd697d55084db059c6b563f70f71f635e935391
Reviewed-on: https://go-review.googlesource.com/c/go/+/194441
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>

src/cmd/go/internal/work/buildid.go

@@ -292,14 +292,19 @@ 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