[IA64] preserve personality flag bits across exec

In its <asm/elf.h> ia64 defines SET_PERSONALITY in a way that unconditionally
sets the personality of the current process to PER_LINUX, losing any flag bits
from the upper 3 bytes of current->personality.  This is wrong. Those bits are
intended to be inherited across exec (other code takes care of ensuring that
security sensitive bits like ADDR_NO_RANDOMIZE are not passed to unsuspecting
setuid/setgid applications).

Signed-off-by: Tony Luck <tony.luck@intel.com>