Restore building on x86-64 when N-FIXNUM-TAG-BITS > 1.

Since commit 6794373d588cef4333fbbb9d04b931ae0a414a7f the build breaks
under this condition when compiling the VOP FIXNUMP/SIGNED-BYTE-64 in
src/compiler/x86-64/type-vops.lisp:

caught WARNING:
  The variable EAX is defined but never used.

Fix this by actually using the declared temporary. While at it, don't
force the temporary into RAX any longer as that offers no advantage over
using any of the other registers (specifically, none of the machine
instructions used in the VOP have a shorter encoding when using RAX).
Better error when LOADing a directory.

Check that the stream does not belong to a directory before reading
anything from it and getting confused.

Fixes lp#1077996.