[Libomptarget] Remove remaining inline assembly from the device RTL (#79922)

Summary:
Recent patches have added some missing intrinsic functions NVPTX. This
patch gets rid of all the remaining uses of inline assembly. The one
change that wasn't directly replaced with a built-in was the `pack` and
`unpack` implementations. However, using the generic C implementation is
equivalent to the output SASS when run through PTXAS.