[forwardport] fix(ios): make sure we can build an iOS release (#598) (#599)

This diff forward ports f47b0c6c16e0cd417e3591358eb85b45962f307d to master.

Original commit message:

- - -

1. we now need to name the framework `.xcframework` otherwise
gomobile refuses to build a new framework for us ¯\_(ツ)_/¯

2. remove duplicate errno definition for iOS (iOS and darwin
are considered the same, therefore we don't need iOS defs)

Reference issue for this PR: https://github.com/ooni/probe/issues/1876

This diff WILL need to be forwardported to master.

internal/netxlite/internal/generrno/main.go

@@ -173,7 +173,7 @@ var Specs = []*ErrorSpec{
 // of the related golang.org/x/sys/$name library.
 func mapSystemToLibrary(system string) string {
 	switch system {
-	case "darwin", "freebsd", "ios", "linux":
+	case "darwin", "freebsd", "linux":
 		return "unix"
 	case "windows":
 		return "windows"
@@ -364,7 +364,6 @@ func writeSystemSpecificTestFile(system string) {
 var SupportedSystems = []string{
 	"darwin",
 	"freebsd",
-	"ios",
 	"linux",
 	"windows",
 }