Bug #1450: Illumos should be buildable with GCC4
uts homebrew offsetofs cause various pointer-cast warnings
Various pieces of the kernel contain homebrew (mostly inlined) implementations of offsetof() which cast down to 32bit results. This causes GCC pointercast warnings, since this will truncate when 64bit.
All of these should use the real offsetof(), which explicitly has a value in size_t (which can then be implicitly truncated as necessary)