1.0.25.1: x86-64 code fixup recording for gc / slash-and-burn

1.0.25.1: x86-64 code fixup recording for gc / slash-and-burn

x86-64 code segments do not have absolute references to within
themselves, nor do they have relative references to without themselves,
making them relocatable without patching.  The GC has long since been
updated to reflect this, but the fixup recording code originally part
of the x86 port had been retained.

  * Removed x86-64 code-object fixup recording code everywhere.

  * Added some commentary to x86iod fixup handling in genesis.