cmd/go: don't scan gccgo standard library packages for imports

In a gccgo installation the standard library sources are not available.

Change-Id: I929f3645e3ac95a1fa7047d6a3d243159a86ba66
Reviewed-on: https://go-review.googlesource.com/c/go/+/281493
Trust: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>
Reviewed-by: Bryan C. Mills <bcmills@google.com>

src/cmd/go/internal/modload/load.go

@@ -1083,14 +1083,21 @@ func (ld *loader) load(pkg *loadPkg) {
 		}
 	}
 
-	imports, testImports, err := scanDir(pkg.dir, ld.Tags)
-	if err != nil {
-		pkg.err = err
-		return
-	}
-
 	pkg.inStd = (search.IsStandardImportPath(pkg.path) && search.InDir(pkg.dir, cfg.GOROOTsrc) != "")
 
+	var imports, testImports []string
+
+	if cfg.BuildContext.Compiler == "gccgo" && pkg.inStd {
+		// We can't scan standard packages for gccgo.
+	} else {
+		var err error
+		imports, testImports, err = scanDir(pkg.dir, ld.Tags)
+		if err != nil {
+			pkg.err = err
+			return
+		}
+	}
+
 	pkg.imports = make([]*loadPkg, 0, len(imports))
 	var importFlags loadPkgFlags
 	if pkg.flags.has(pkgInAll) {