increase range of relative jumps by aligning blocks to 8-byte boundaries

* libguile/objcodes.c (OBJCODE_COOKIE): Bump again, as our jump offsets
  are now multiplied by 8.

* libguile/vm-i-system.c (BR): Interpret the 16-bit offset as a relative
  jump to the nearest 8-byte-aligned block -- increasing relative jump
  range from +/-32K to +/-240K.
  (mvra): Do the same for the mvra jump.

* libguile/vm.c (really_make_boot_program): Align the mvra.

* module/language/assembly.scm (align-block): New export, for aligning
  blocks.

* module/language/assembly/compile-bytecode.scm (write-bytecode): Emit
  jumps to the nearest 8-byte-aligned block. Effectively our range is 18
  bits in either direction. I would like to do this differently -- have
  long-br and long-br-if, and all the other br instructions go to 8 bits
  only. But the assembler doesn't have an appropriate representation to
  allow me to do this yet, so for now this is what we have.

* module/language/assembly/decompile-bytecode.scm (decode-load-program):
  Decode the 19-bit jumps.

module/language/glil/compile-assembly.scm

@@ -340,11 +340,12 @@
           (error "unknown module var kind" op key)))))
 
     ((<glil-label> label)
-     (values '()
-             bindings
-             source-alist
-             (acons label addr label-alist)
-             object-alist))
+     (let ((code (align-block addr)))
+       (values code
+               bindings
+               source-alist
+               (acons label (addr+ addr code) label-alist)
+               object-alist)))
 
     ((<glil-branch> inst label)
      (emit-code `((,inst ,label))))