[IA64] abstract SAL_CALL wrapper to allow other firmware entry points

SAL_CALL() always calls through the ia64_sal function pointer.  I am adding
new functionality that needs the same conventions as SAL_CALL (FP regs
saved/restored, sal_lock acquired, etc), but doesn't use the ia64_sal
function pointer.

This patch pulls the body of SAL_CALL out into a new "IA64_FW_CALL" that
takes care of these calling conventions, but allows the caller to specify
either ia64_sal or some other firmware entry point.

Signed-off-by: Bjorn Helgaas <bjorn.helgaas@hp.com>
Signed-off-by: Len Brown <len.brown@intel.com>
Signed-off-by: Tony Luck <tony.luck@intel.com>