[Builtins] Implement f2h/h2f by jumping to trunc/extend.

Follow-up to r237161; seems like we can't use aliases, but we
can do better than duplicating the bodies, especially when that
body, after inlining, isn't as small as it looks.

Better approaches welcome.  Perhaps the best thing is just to have
an #ifndef __APPLE__ over the GNUEABI names, since they're not used
there.

llvm-svn: 237323