mpi/longlong: provide generic implementation using double word type

* configure.ac: Add check for 'unsigned __int128'.
* mpi/longlong.h (UDWtype): Define for 32-bit or 64-bit when
'unsigned long long' or 'unsigned __int128' is available.
(add_ssaaaa, sub_ddmmss, umul_ppmm, udiv_qrnnd) [UDWtype]: New.

New generic longlong.h implementation by using 'unsigned long long'
on 32-bit and 'unsigned __int128' on 64-bit (for new architectures like
RISC-V).

• Signed-off-by: Jussi Kivilinna <jussi.kivilinna@iki.fi>