New version for new compiler. Many changes due to new register

New version for new compiler.  Many changes due to new register
calling convention and preservation.

95/06/28
Fixed fixnum_lsh to compute correct values for negative shifts.

95/03/28
Fixed off-by-1 error at end of ADDIB chain at new_interrupt_common.

94/12/13
Tidied up generic arithmetic some more.  Flonum allocation is now
`open-coded', trimming a few cycles of the operation.

Re-arranged the builtin table as some things had moved position.

94/12/12

. fixed generic multiply to do fixnum*fixnum case (it was calling the
  handler procedure, reducing performance by a factor of 7).
. fixed generic multiply to return exact 0 when multiplying a flonum
  by exact 0.
. tidied other generic operations