* include/std/numeric (midpoint(T, T)): Change implementation for
	floating-point types to avoid incorrect rounding of denormals.
	* testsuite/26_numerics/midpoint/floating.cc: Add check for correct
	rounding with denormals.
	* testsuite/26_numerics/gcd/gcd_neg.cc: Adjust dg-error line numbers.
	* testsuite/26_numerics/lcm/lcm_neg.cc: Likewise.

From-SVN: r272616