cmd/go: update -DGOPKGPATH to use current pkgpath encoding

This will need to be done in the gc version too, probably more cleverly.
This version will ensure that the next GCC release works correctly
when using the GCC version of the go tool.

Updates golang/go#37272

Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/219817

gcc/go/gofrontend/MERGE

@@ -1,4 +1,4 @@
-8505defaa91ecc5b42afd02eb335981e8b02b288
+d5d00d310ec33aeb18f33f807956ec0c4eeea6bb
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.

libgo/go/cmd/go/internal/work/gccgo.go

@@ -596,14 +596,28 @@ func gccgoPkgpath(p *load.Package) string {
 	return p.ImportPath
 }
 
+// gccgoCleanPkgpath returns the form of p's pkgpath that gccgo uses
+// for symbol names. This is like gccgoPkgpathToSymbolNew in cmd/cgo/out.go.
 func gccgoCleanPkgpath(p *load.Package) string {
-	clean := func(r rune) rune {
+	ppath := gccgoPkgpath(p)
+	bsl := []byte{}
+	changed := false
+	for _, c := range []byte(ppath) {
 		switch {
-		case 'A' <= r && r <= 'Z', 'a' <= r && r <= 'z',
-			'0' <= r && r <= '9':
-			return r
+		case 'A' <= c && c <= 'Z', 'a' <= c && c <= 'z',
+			'0' <= c && c <= '9', c == '_':
+			bsl = append(bsl, c)
+		case c == '.':
+			bsl = append(bsl, ".x2e"...)
+			changed = true
+		default:
+			encbytes := []byte(fmt.Sprintf("..z%02x", c))
+			bsl = append(bsl, encbytes...)
+			changed = true
 		}
-		return '_'
 	}
-	return strings.Map(clean, gccgoPkgpath(p))
+	if !changed {
+		return ppath
+	}
+	return string(bsl)
 }