libstdc++: Prefer double to long double in std::shuffle_order_engine

libstdc++: Prefer double to long double in std::shuffle_order_engine

The transition algorithm for std::shuffle_order_engine uses long double
to ensure that the value (max() - min() + 1) can be accurately
represented, to avoid bias in the shuffling. However, when the base
engine's range is small enough we can avoid slower long double
arithmetic by using double. For example, long double is unnecessary for
any base engine returning 32-bit values.

This makes std::knuth_b::operator() about 15% faster on x86_64, and
probably even more on targets where long double uses soft-float.

libstdc++-v3/ChangeLog:

	* include/bits/random.h (independent_bit_engine): Fix typo
	in comment.
	(shuffle_order_engine): Fix incorrect description in comment.
	* include/bits/random.tcc (__representable_as_double
	(__p1_representable_as_double): New helper functions.
	(shuffle_order_engine::operator()): Use double for calculation
	if (max() - min() + 1) is representable as double.
	* testsuite/26_numerics/random/pr60037-neg.cc: Adjust dg-error
	line number.