include/qemu/atomic.h: default to __atomic functions

The __atomic primitives have been available since GCC 4.7 and provide
a richer interface for describing memory ordering requirements. As a
bonus by using the primitives instead of hand-rolled functions we can
use tools such as the ThreadSanitizer which need the use of well
defined APIs for its analysis.

If we have __ATOMIC defines we exclusively use the __atomic primitives
for all our atomic access. Otherwise we fall back to the mixture of
__sync and hand-rolled barrier cases.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <1453976119-24372-4-git-send-email-alex.bennee@linaro.org>
[Use __ATOMIC_SEQ_CST for atomic_mb_read/atomic_mb_set on !POWER. - Paolo]
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

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