ENABLE_PERL64 multiarch builds
illumos-gate needs to support distros with multiarch perl implementations such as omnios.
illumos-omnios has done the primary work, but in addition shadow compiling needs to
have E_ATTRIBUTE_UNKNOWN and E_ATTRIBUTE_PARAM_UNDEFINED stiffled
in absence of the perl patch to sunpro when primarily built with gcc (patchset used
in OI/hipster but not yet, for example, in omnios itself).
This is particularly useful for gate developers using omnios when wishing to ONU and
actually use their, or others, work (and is frequently requested by advocates prior to RTI)