Make entries point to _after_ the PC offset.

Make entries point to _after_ the PC offset.

This saves a jump in closure headers, and makes non-closure entries
have a nice PC offset of 0 rather than an awkward PC offset of 8.
However, this causes all indirect calls to have an additional offset
of -8 in the addressing mode -- not clear yet how much this hurts.

WARNING: This changes the amd64 compiled code interface so that new
compiled code requires a new microcode and vice versa.  Further, you
must set compiler:cross-compiling? to #t to compile the system,
because compiled code block offsets are now in a different place
relative to compiled entries, so the native fasdumper of an old
microcode can't handle compiled entries produced by a new compiler.