gallivm: fix half to float conversions with llvm 11

LLVM 11 removes the intrinsic for half to float conversion, so use the fpext function instead. This function actually works now with half to float, albeit a quick experiment showed at least the x86 backend cannot lower it itself if the cpu doesn't support it natively and tries to call external library, which crashes (and would be very slow anyway as it would be lowered to scalar code), so for now only use it where we previously used the f16c intrinsic.

Closes: #2603 (closed)