Simplify x86-64 utility interface, and nuke all x87 code from it.

Simplify x86-64 utility interface, and nuke all x87 code from it.

The calling convention for microcode utilities now more closely
matches the standard calling convention for C on the AMD64, which
reduces the amount of code needed in scheme_to_interface.  The i386
hack to split the assembly hook addresses between negative addresses
and positive addresses is no more; most of what little benefit it
added on the i386, it ceased to add on x86-64, since no hooks fit in
positive byte offsets from the register block start any longer, and
only sixteen would fit in negative byte offsets.  Perhaps later it
would be worth deciding which sixteen deserve byte offsets (e.g.,
scheme-to-interface certainly would), but this is simpler for now.
Note that there is still a space advantage to invoking a hook (jump
to an offset from Registers) over invoking an interface (move its
number to AL, jump to scheme-to-interface), since invoking an
interface requires going through a hook anyway.

These change serves mainly to simplify the code, not to improve
performance, although shuffling the calling convention may improve
performance as a bonus.

The x87 code appears to have been causing problems, and will later
be replaced by the AMD64's 128-bit media (SSEn) instructions anyway.
The compiler still has some x87 vestiges left in it, but not in any
code that is hit, and nearly all of it is commented out now.

*** NOTE:  Since this changes the calling convention of utilities
from compiled code, it breaks all existing x86-64 .com files, which
you must recompile with the new compiler.  This means that you can't
straightforwardly rebuild Scheme the usual way from a prior x86-64
installation.  The easiest way to proceed is to start again from a
LIARC snapshot and rebootstrap the x86-64 code.  Expect more of this
to come; in particular, since we now have a reasonable number of
machine registers, it may be worthwhile to assign one to be the
return value register.