[nfc][libomptarget] Use __kmpc_atomic_add inRefactor target_impl.cu
Use the __kmpc_impl_atomic wrapper functions consistently. Alternatively, target_impl.cu_add instead of atomicAdd to match the rest of the file.
Alternatively, target_impl.cu could use the cuda functions directly. Using a mixture in this file was an
file was an oversight, happy to resolve in either direction.
Also rRemoved some comments that look outdated in passing..
Call __kmpc_impl_unset_lock directly to avoid a redundant diagnostic and remove an implict
dependency on interface.h