Replace deliberate division by zero with _gcry_divide_by_zero.

* mpi/mpi-pow.c: Replace 1 / msize.
* mpi/mpih-div.c: Replace 1 / dsize.
* src/misc.c: Add _gcry_divide_by_zero.

1. Division by zero doesn't "provoke a signal" on architectures like PowerPC.

2. C compilers like clang will optimize away these divisions, even though the code tries "to make the compiler not remove" them.

This patch redirects these cases to _gcry_divide_by_zero.