[PATCH] Allow __fp16 as a function arg or return type for AArch64

[Tim Northover]

Hi Oliver,

I don't think the right way to handle this is by mapping __fp16 to
different types depending on the context. It's a hack in the front-end
to work around some not particularly difficult backend problems.

If we need __fp16 to map to half, we should make the extra effort so
that it can be a blanket change. It's the right direction to go in
anyway.

Cheers.

Tim.