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

Change-Id: I5636bec7268bcd41994c2e2262f8293cb95c0cd4
Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/219817
Reviewed-by: Than McIntosh <[email protected]>

Author: ianlancetaylor

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)
 }