the only one we actually use).  This allows REAL_EXEC fragments to
contain proper references to constraints (e.g. "%w0"), rather than having
to force the use of specific registers.

Note that the "num_constraints" parameter is now completely obsolete, and
that we can probably reduce the syntax to something like
  __asm__ __volatile__ ( REAL_CODE ( "asm statements" )
                         : output constraints
                         : input constraints
                         : clobber );
which would look much more natural, and avoid the need to always specify
a clobber list.


Add userptr_t to libkir.h, to allow it to at least compile.