[lldb] Support Rosetta registers in crashlog.py

Rosetta crashlogs can have their own thread register state. Unlike the
other registers which ware directly listed under "threadState", the
Rosetta registers are nested under their own key in the JSON, as
illustrated below:

  {
      "threadState":
      {
          "rosetta":
          {
              "tmp2":
              {
                  "value": 4935057216
              },
              "tmp1":
              {
                  "value": 4365863188
              },
              "tmp0":
              {
                  "value": 18446744073709551615
              }
          }
      }
  }