I've committed this to gomp4 branch.  It replaces the regular builtins 
__builtin_GOACC_nid/__builtin_GOACC_id with internal functions IFN_OACC_DIM_SIZE 
and IFN_OACC_DIM_POS -- moving further away from the PTX-specific naming of id & 
nid.  These functions should never turn into library calls or be accessible by 
the user.

A later patch will optimize the OACC_DIM_SIZE function in the oacc-xform pass.

nathan