Add unix signal hit counts to the target statistics.

Android and other platforms make wide use of signals when running applications and this can slow down debug sessions. Tracking this statistic can help us to determine why a debug session is slow.

The new data appears inside each target object and reports the signal hit counts:

      "signals": [
        {
          "SIGSTOP": 1
        },
        {
          "SIGUSR1": 1
        }
      ],

Differential Revision: https://reviews.llvm.org/D112683