powerpc/xmon: Show task->thread.regs in process display

Show the address of the tasks regs in the process listing in xmon. The
regs should always be on the stack page that we also print the address
of, but it's still helpful not to have to find them by hand.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200520111740.953679-1-mpe@ellerman.id.au