factor: fix bug with 128-bit uintmax_t

factor: fix bug with 128-bit uintmax_t

On so-far-only-theoretical platforms with 128-bit uintmax_t,
'factor' would misbehave by not factoring enough.
Work around the bug (at a performance cost)
and document the issue.  I hope someone with
more time and expertise can fix the performance cost
that this introduces.
To reproduce the correctness bug, build with
'gcc -DUSE_INT128 -DEXHIBIT_INT128_BUG';
'make check' should fail due to the new test case.
* src/factor.c (USE_INT128): New macro.
(wide_uint, wide_int, W_TYPE_SIZE, WIDE_UINT_MAX):
Define to proper values if USE_INT128.
(prime_p) [!EXHIBIT_INT128_BUG]: Work around bug
with 128-bit wide_uint, at some performance cost.
* tests/factor/factor.pl (bug-with-128-bit-uintmax_t):
New test.