Remove comment that no target supports 128-bit IEEE floats

The soon-to-be-committed SystemZ port uses 128-bit IEEE floats.
MIPS64 GNU/Linux does too (albeit with unusual NaNs).


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@181016 91177308-0d34-0410-b5e6-96231b3b80d8