Simplify non-native 64x64-bit multiplication in int128.h.

Simplify non-native 64x64-bit multiplication in int128.h.

In the non-native code in int128_add_int64_mul_int64(), use signed
64-bit integer multiplication instead of unsigned multiplication for
the first three product terms. This simplifies the code needed to add
each product term to the result, leading to more compact and efficient
code. The actual performance gain is quite modest, but it seems worth
it to improve the code's readability.

Author: Dean Rasheed <dean.a.rasheed@gmail.com>
Reviewed-by: John Naylor <johncnaylorls@gmail.com>
Discussion: https://postgr.es/m/CAEZATCWgBMc9ZwKMYqQpaQz2X6gaamYRB+RnMsUNcdMcL2Mj_w@mail.gmail.com