This fixes invalid and inconsistent results for the Fixnum*Fixnum case
where the result of the multiplication does not fit in 64-bit
on OpenBSD/mips64.  For example:

  $  for x in 1 23; do ruby31 -e 'p(54306000000000*86400)'; done
  14409380628474329524
  11410664325873689790

Cases where an argument was Bignum, as well as cases where the result
of the multiplication fits in 64-bit are fine:

  $ for x in 1 23; do ruby31 -e 'p(54306000*86400)'; done
  4692038400000
  4692038400000

  $ for x in 1 23; do ruby31 -e 'p(5430600000000000000000*86400)'; done
  469203840000000000000000000
  469203840000000000000000000

This was originally discovered by running the tests for the openssl gem
on OpenBSD/mips64 and having one test fail for a date far in the future.
I eventually traced this to the generic multiplication issue.

The underlying cause is using the int128_t type. This avoids use of the
int128_t type in this case, falling back to the slower conversion code,
which in the overflow case, turns the Fixnums into Bignums, then
performs the multiplication.