Added silent alarm function to internal chronometer

This lets you set an alarm if you have the internal chronometer
installed. It has an effective volume of 20, compared with 12 on the
normal alarm.

It is also completely silent, so won't attract unwanted attention.

This changes the internal chronometer from a novelty (it was neat, but
you need a watch anyway if you want to set alarms) into something
that's has a benefit over the easier alternative of taking a watch off
a dead guy.
Added description of internal chronometer alarm function to its description
Merge pull request #7918 from rluddy/bionic-alarm-clock

Adds silent alarm clock to internal chronometer