Improve some test cases for fixnum-width independence
* In arith.pure.lisp, some test cases were defined in terms of
n-lowtag-bits instead of n-fixnum-tag-bits.
* In sb-aclrepl, the bignum used for testing fit within an
(signed-byte 63), the largest possible fixnum type on a 64-bit system.
* Thanks to Paul Khuong for finding and fixing the arith.pure.lisp