[IA64] Liberate the signal layer from IA64 assembler

Currently IA64 has a assembler implementation of sigrtprocmask.
Having a single architecture implement this in assembler language
is a serious maintenance problem that inhibits further evolution of the
signal subsystem.  Everyone who wants to do deep changes to signals
would need to learn that assembler language.

Whatever performance improvements IA64 gets from this it cannot be worth
the price in maintainability.

We have some locking problems in signal that need to be fixed,
but this roadblock needs to be removed first.

So just disable the special assembler IA64 implementation and fall back to a
normal syscall there.

Acked-by: Matt Fleming <matt.fleming@intel.com>
Signed-off-by: Andi Kleen <andi@firstfloor.org>
Signed-off-by: Tony Luck <tony.luck@intel.com>