powerpc: Add 'show frame <addr>' to show an arbitrary trap frame

This lets us print, for example, the user's trap frame when a panic occurs.
The frame address is given in the backtrace at the trap point, which can
then be passed to 'show frame'. This is useful for debugging as it can show
inputs that lead to a panic or fault. It can also be used to print trap
frames from other CPUs that get stuck.

i386 already has a similar command, but no others do.