cmd/cgo: fix typo in gccgo name mangling recipe

The code to implement new-style gccgo name mangling had a recipe that
    didn't quite match the one in the compiler (incorrect handling for
    '.'). This showed up as a failure in the gotools cgo test if the
    directory containing the test run included a "." character.
    
    Reviewed-on: https://go-review.googlesource.com/c/147917

From-SVN: r265981

gcc/go/gofrontend/MERGE

@@ -1,4 +1,4 @@
-da8c968474690d1e77442ac3361b2302ea8e1f36
+559fae430b81595efe151222385192a07a9fc3c3
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.

libgo/go/cmd/cgo/out.go

@@ -1261,7 +1261,7 @@ func gccgoPkgpathToSymbolNew(ppath string) string {
 	for _, c := range []byte(ppath) {
 		switch {
 		case 'A' <= c && c <= 'Z', 'a' <= c && c <= 'z',
-			'0' <= c && c <= '9', '_' == c:
+			'0' <= c && c <= '9', c == '_', c == '.':
 			bsl = append(bsl, c)
 		default:
 			changed = true