[OpenCL] Move most _explicit atomics into multiclass; NFC

This will simplify future conditionalization for OpenCL 3.0
optionality of atomic features.

The only set of atomic functions not using the multiclass is
atomic_compare_exchange_strong/weak, as these don't fit the common
pattern due to having 2 MemoryOrder arguments.

(cherry picked from commit d97a4dfe)