libstdc++: Avoid divide by zero in default template arguments

libstdc++: Avoid divide by zero in default template arguments

My previous attempt to fix this only worked when m is a power of two.
There is still a bug when a=00 and !has_single_bit(m).

Instead of trying to make _Mod work for a==0 this change ensures that we
never instantiate it with a==0. For C++17 we can use if-constexpr, but
otherwise we need to use a different multipler. It doesn't matter what
we use, as it won't actually be called, only instantiated.

libstdc++-v3/ChangeLog:

	* include/bits/random.h (__detail::_Mod): Revert last change.
	(__detail::__mod): Do not use _Mod for a==0 case.
	* testsuite/26_numerics/random/linear_congruential_engine/operators/call.cc:
	Check other cases with a==0. Also check runtime results.
	* testsuite/26_numerics/random/pr60037-neg.cc: Adjust dg-error
	line.