phobos unittests not passing with dmd built by clang

[dennis luehring]

> The behaviour of the sign bit is completely specified in the x86
> manuals. As that link says, the C standard got it wrong in a couple of
> places.
> The one thing which is implementation specific is that on Intel, an 80
> bit load of a signalling NaN doesn't raise an exception, whereas it does
> on AMD. I don't know what Via does.

just a question:

would it be a good idea to test this platform specific behavior
in the phobos unit-tests?

produce "an 80 bit load of a signalling NaN"
check:
   intel: do not raise
   amd: do raise
   via: ...

maybe only for showing that D is aware of this special cases?