-ftime-report might wrap

[Unruh, Erwin]

Hello,

I stumbled over a problem in timevar.c, function timevar_accumulate. When 
USE_CLOCK is set on a 32-bit system, the counter silently wraps around. This 
can result in negative time values, or some time being ignored (because one 
pass uses more than 72 minutes of time).

A check like 
        if (stop_time->user < start_time->user)
could detect that problem. (Unless a pass uses 72 minutes in one chunk.)

        Erwin