An info table with an SRT normally looks like this:
StgWord64 srt_offset StgClosureInfo layout StgWord32 layout StgWord32 has_srt
But we only need 32 bits for srt_offset on x86_64, because the small
memory model requires that code segments are at most 2GB. So we can
optimise this to
StgClosureInfo layout StgWord32 layout StgWord32 srt_offset
saving a word. We can tell whether the info table has an SRT or not,
because zero is not a valid srt_offset, so zero still indicates that
there's no SRT.