Use indirection for entry points on amd64.

Use indirection for entry points on amd64.

A compiled entry is now a tagged address A pointing to a 64-bit word
W such that A + W points to instruction to execute.

This adds a memory indirection overhead to unknown procedure calls,
but it has the effect that consing a closure only involves writing
data memory, not instruction memory that must be reloaded into the
CPU's instruction cache.

WARNING: This changes the amd64 compiled code interface, so you'll
have to build a new compiler running on an old microcode and use that
to compile a new system afresh.