[IA64] unwind: remove preprocesser noise, and correct comment

The expression in the comment was mis-bracketted.
There was also no need to introduce a coding-style breaking
macro for a single use, so just made it a static const.

Signed-off-by: Phil Carmody <ext-phil.2.carmody@nokia.com>
Signed-off-by: Tony Luck <tony.luck@intel.com>