dtrace/profile: Set t_dtrace_trapframe for profile probes

profile provider probes fire in the context of a timer interrupt. Thus,
the "regs" action can make use of the interrupt trap frame to get
register values when the interrupt happened in kernel mode. Make that
trap frame available when possible so that "regs" works more or less as
it already does with the fbt and kinst providers.

MFC after: 1 week