net/ip, syscall/errno: Reduce code duplication by switching to internal/itoa.

internal/itoa wasn't around back in go 1.12 days when tinygo's syscall/errno.go was written.
It was only added as of go 1.17 ( https://github.com/golang/go/commit/061a6903a232cb868780b )
so we have to have an internal copy for now.
The internal copy should be deleted when tinygo drops support for go 1.16.

FWIW, the new version seems nicer.
It uses no allocations when converting 0,
and although the optimizer might make this moot, uses
a multiplication x 10 instead of a mod operation.

src/syscall/errno.go

@@ -1,5 +1,7 @@
 package syscall
 
+import "internal/itoa"
+
 // Most code here has been copied from the Go sources:
 //   https://github.com/golang/go/blob/go1.12/src/syscall/syscall_js.go
 // It has the following copyright note:
@@ -18,7 +20,7 @@ package syscall
 type Errno uintptr
 
 func (e Errno) Error() string {
-	return "errno " + itoa(int(e))
+	return "errno " + itoa.Itoa(int(e))
 }
 
 func (e Errno) Temporary() bool {