Main x86_64 hacking: we have a problem on this arch where binutils
can't generate 64-bit relative relocations (R_X86_64_PC64), which many
of our info-table fields are.  So far we've been hacking around it by
putting everything in the text section, but I've decided to adopt
another approach: we'll use explicit 32-bit offset fields on this
platform instead.  This is safe in the default "small" memory model
where all symbols are guaranteed to be in the lower 2Gb of the address
space.

NCG changes coming; mangler changes are probably required too.