generic atomicops: promote Acquire_Store() and Release_Load() to use SEQ_CST fence

__atomic_store_n() cannot take a memory model argument of
__ATOMIC_ACQUIRE, and __atomic_load_n() cannot take a memory model
argument of __ATOMIC_RELEASE, per the GCC documentation:

    https://gcc.gnu.org/onlinedocs/gcc-4.9.1/gcc/_005f_005fatomic-Builtins.html

On Clang this generates a -Watomic-memory-ordering warning.

Promote the fences in Acquire_Store() and Release_Load() to the stronger
__ATOMIC_SEQ_CST memory model, which ought to be safe.

Note that there are no actual uses of Acquire_Store() or Release_Load()
in protobuf, though.

This follows the TSAN atomicops implementation, which also uses SEQ_CST
fences for these functions.

(Fixes #25.)

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