Fix std::midpoint for denormal values

Fix std::midpoint for denormal values

Backport from mainline
2019-06-25  Jonathan Wakely  <jwakely@redhat.com>

	* include/std/numeric (midpoint(T, T)): Avoid std::abs in constexpr
	function.

Backport from mainline
2019-06-24  Jonathan Wakely  <jwakely@redhat.com>

	* 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: r273829