r6rs: fix `(fl+)' and `(fl*)'

Closes PR 13922
fix `arithmetic-shift' with shifts close to `(- (expt 2 31))' on 64-bit

The bug was caused by an accidental cast of a 64-bit integer to a
32-bit integer.

Closes PR 13923
JIT repair for `(/ (most-negative-fixnum) -1)'

When I added JIT-inling of '/' for fixnums, I somehow forgot to
handle the special case that is already handled for `quotient'.
r6rs: fix `div' and `mod'

Closes PR 13925