[LLDB] Format lldb-server's target XML

So that the XML isn't one giant line. Which wasn't
a problem for lldb but was for me trying to troubleshoot
it using the logs.

It now looks like:
```
<?xml version="1.0"?>
<target version="1.0">
  <architecture>aarch64</architecture>
  <feature>
    <...>
    <reg name="fpcr" .../>
  </feature>
</target>
```

Reviewed By: labath

Differential Revision: https://reviews.llvm.org/D134035