This adds ToXML methods to encode RegisterFlags and its fields into XML
according to GDB's target XML format:
https://sourceware.org/gdb/onlinedocs/gdb/Target-Description-Format.html#Target-Description-Format
lldb-server does not use libXML to build XML, so this follows the
existing code that uses strings. Indentation is used so the result is
still human readable.
```
<flags id=\"Foo\" size=\"4\">
<field name=\"abc\" start=\"0\" end=\"0\"/>
</flags>
```
This is used by lldb-server when building target XML, though no one sets
any fields yet. That'll come in a later commit.