MASK_GNU_AS == MASK_GAS

[Fred Fish]

It appears that the ia64 port introduced the internal define
MASK_GNU_AS that is used the same was as the historical MASK_GAS
define.  There was some discussion of this about 5 years ago as part
of a larger discussion about possible user level changes.

It would seem to be a pretty simple change to just make them
consistent for internal gcc sources without diddling with the user
visible issues.

-Fred