Add stub implementations of the missing C++11 math functions.
These are weak and so can be replaced by other versions in applications
that choose to do so, and will give a linker warning when used so that
applications that rely on the extra precision can avoid them.

Note that since the C/C++ specs only guarantee that long double has
precision equal to double, code that actually relies on these functions
having greater precision is unportable at best and broken at worst.