[NVPTX] Implement `__builtin_readcyclecounter` on NVPTX (#81344)

Summary:
This patch simply states that `__builtin_readcyclecounter` is legal on
NVPTX and makes it  return the value from the `clock64` sreg. The timer
intrinsics are marked as having side effects, which is desireable for
timing primitives and required to pattern match the instrinic DAG.