Fix non-portable use of std::abs(double) in constexpr function

Fix non-portable use of std::abs(double) in constexpr function

Although libstdc++ adds 'constexpr' to its std::abs(floating-point)
overloads (as a non-conforming extension), those overloads are not used
if the target libc provides them, which is the case on Solaris.

The fix is to avoid std::abs and simply apply the negation when needed.

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

From-SVN: r272653