ARM: rework "arm reg" output for new mode

ARM: rework "arm reg" output for new mode

Change the layout to show the "Secure Monitor" registers too,
when they're present.

Instead of lining registers for each of six (or seven) modes up
in adjacent vertical columns, display each mode's registers (or
shadows) in a single block, avoiding duplicate value displays.

This also lets us shrink the line length to fits in standard 80
character lines ... six or seven 18-character columns can't fit.

Relabel "r13" as "sp", so it's more meaningful.

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>