[IA64] Cleanup: Move is_platform_hp_ski() from xenmisc.c to xensetup.c

[IA64] Cleanup: Move is_platform_hp_ski() from xenmisc.c to xensetup.c

- only caller is start_kernel
- change to static __init
- also move running_on_sim to xensetup.c, and change it from unsigned
  long to int, since it's just a boolean
- declare running_on_sim in config.h near some other externs

Tested by building, booting, starting a PV guest on rx2620.

Signed-off-by: Aron Griffis <aron@hp.com>