icu: remove BR2_ARCH_HAS_ATOMICS dependency

The BR2_ARCH_HAS_ATOMICS was added because on ARC, atomic instructions
may not be provided by the architecture and therefore the compiler
does not provide the __sync_*() built-ins.

However, since then, icu was changed and is now able to use C++11
atomics, or even no atomic operations at all. In fact, icu will:

 * If possible, it will use C++11 atomics, which internally rely on
   the __atomic built-ins. These are available since gcc 4.7, and all
   architectures provide it. On some architectures, you *must* link
   with libatomic, on some other architectures, they are available
   built-in, but in all cases, linking against libatomic does not
   harm. Thanks to this, even ARC with no atomic support (which was
   the original reason for adding the BR2_ARCH_HAS_ATOMICS) dependency
   builds fine, provided -latomic is added to LIBS.

 * If C++11 atomics are not available, then it falls back to
   __sync_*() built-ins, which allows compilers older than 4.7 to be
   supported.

 * If really no atomic mechanism is available, then it falls back to a
   basic implementation based on a mutex.

Conclusion:

 - The BR2_ARCH_HAS_ATOMICS dependency is no longer needed.

 - We need to link with -latomic when gcc >= 4.7 is used.

Note that reverse dependencies of icu are also changed accordingly.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>