How to deal with systems where real == double

[Johannes Pfau]

How do we want to deal with systems which have no floating point type
bigger than double?

* What's the proper check to detect this in druntime / phobos?
  (real.sizeof == double.sizeof) or (real.mant_dig == double.mant_dig)?

* Should the long double functions (tanl, cosl) be available from
  druntime? If so we need to alias them. Glibc does not provide the
  long double functions on a target without long double.

* What about phobos functions overloads using "real"? Should those be
  disabled completely or should they call the "double" equivalent
  function?