toolchain: introduce BR2_TOOLCHAIN_HAS_LIBATOMIC

Until now, we were assuming that whenever you have gcc 4.8, libatomic
is available. It turns out that this is not correct, since libatomic
will not be available if thread support is disabled in the toolchain.

Therefore, __atomic_*() intrinsics may not be available even if the
toolchain uses gcc 4.8.

To solve this problem, we introduce a BR2_TOOLCHAIN_HAS_LIBATOMIC
boolean, which indicates whether the toolchain has libatomic. It is
the case when you are using gcc >= 4.8 *and* thread support is
enabled. We then use this new BR2_TOOLCHAIN_HAS_LIBATOMIC to define
BR2_TOOLCHAIN_HAS_ATOMIC.

As explained in the comment, on certain architectures, libatomic is
technically not needed to provide the __atomic_*() intrinsics since
they might be all built-in. However, since libatomic is only absent in
non-thread capable toolchains, it is not worth making things more
complex for such seldomly used configuration.

Note that we are introducing the intermediate
BR2_TOOLCHAIN_HAS_LIBATOMIC option because it will be useful on its
own for certain packages.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
[Thomas: improve Config.in comment using a suggestion from Yann.]

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