ARM: fix @llvm.convert.from.fp16 on softfloat targets.

We need to make sure we use the softened version of all appropriate operands in
the libcall, or things go horribly wrong. This may entail actually executing a
1-stage softening.
ARM: add __aeabi_d2h for truncation on AEABI systems

ARM does actually define the name for this conversion, so we should use it on
"-eabi" platforms.