Make various fixes to <stdatomic.h>.
Make various fixes to <stdatomic.h>.

- According to the standard, memory_order is a type. Use a typedef.

- atomic_*_fence() and atomic_flag_*() are described by the standard as
  functions. Use inline functions to implement them.

- Only expose the atomic_*_explicit() functions in kernel space. We
  should not use the short-hand functions, as they will always use
  memory_order_seq_cst.

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