cmd/go: ignore errors from go/build for standard packages

The go/build package does not know that gccgo's standard packages don't
    have source, and will report an error saying that it can not find them.
    Work around that in the cmd/go sources, since the go/build sources don't
    currently have a list of standard packages.
    
    This should get a real fix in the master sources, somehow.
    
    Fixes golang/go#16701.
    
    Reviewed-on: https://go-review.googlesource.com/27052

From-SVN: r239486

gcc/go/gofrontend/MERGE

@@ -1,4 +1,4 @@
-24e0c4c98e0614b1892316aca787f1c564f2d269
+affb1bf5fcd7abf05993c54313d8000b93a08d4a
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.

libgo/go/cmd/go/pkg.go

@@ -763,6 +763,13 @@ var cgoSyscallExclude = map[string]bool{
 func (p *Package) load(stk *importStack, bp *build.Package, err error) *Package {
 	p.copyBuild(bp)
 
+	// When using gccgo the go/build package will not be able to
+	// find a standard package.  It would be nicer to not get that
+	// error, but go/build doesn't know stdpkg.
+	if runtime.Compiler == "gccgo" && err != nil && p.Standard {
+		err = nil
+	}
+
 	// The localPrefix is the path we interpret ./ imports relative to.
 	// Synthesized main packages sometimes override this.
 	p.localPrefix = dirToImportPath(p.Dir)