powerpc/xmon: Add xmon command to dump process/task similar to ps(1)

[Douglas Miller]

    Add 'P' command with optional task_struct address to dump all/one task's
    information: task pointer, kernel stack pointer, PID, PPID, state
    (interpreted), CPU where (last) running, and command.
    
    Introduce XMON_PROTECT macro to standardize memory-access-fault
    protection (setjmp). Initially used only by the 'P' command.