The handling of varargs is platform-dependent.  Assar Westerlund
stared at the problem for a while and deduces the following table:

vers / compiler         cc              gcc
----------------------------------------------------------------------
1.3                     a0, offset      __base, __offset
2.0                     _a0, _offset    __base, __offset

The current code should handle both cc and gcc versions, provided
you configure for the correct compiler.