qemu/atomic: Drop special case for unsupported compiler

qemu/atomic: Drop special case for unsupported compiler

Since commit efc6c070aca ("configure: Add a test for the
minimum compiler version") the minimum compiler version
required for GCC is 4.8, which has the GCC BZ#36793 bug fixed.

We can safely remove the special case introduced in commit
a281ebc11a6 ("virtio: add missing mb() on notification").

With clang 3.4, __ATOMIC_RELAXED is defined, so the chunk to
remove (which is x86-specific), isn't reached either.

Signed-off-by: Philippe Mathieu-Daudé <[email protected]>
Reviewed-by: Marc-André Lureau <[email protected]>
Message-Id: <20201210134752[email protected]>
Signed-off-by: Paolo Bonzini <[email protected]>