libgo: better cmd/cgo handling for '.' in pkgpath

    
    Updates cgo's gccgoPkgpathToSymbolNew() to bring it into
    conformance with the way that gccgo now handles package
    paths with embedded dots (see CL 200838). See also
    https://gcc.gnu.org/PR61880, a related bug. This CL is a
    copy of CL 207957 in the main Go repo.
    
    Updates golang/go#35623.
    
    Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/207977

From-SVN: r278470

gcc/go/gofrontend/MERGE

-25d5e9dca49ad3f49393b254dd87f8df51487c65
+245904ac148f15db530fb8d50f526c47d08024c5
 
 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

@@ -1318,8 +1318,10 @@ 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 == '_', c == '.':
+			'0' <= c && c <= '9', c == '_':
 			bsl = append(bsl, c)
+		case c == '.':
+			bsl = append(bsl, ".x2e"...)
 		default:
 			changed = true
 			encbytes := []byte(fmt.Sprintf("..z%02x", c))