Add the C99 hex-float assembly syntax to our extension document.

As Ben pointed out, GAS doesn't support this syntax so we should give at least
some warning that it might not be portable.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@188377 91177308-0d34-0410-b5e6-96231b3b80d8