ceph.spec.in: BuildRequires gcc-toolset-11-libatomic-devel for aarch64

ceph.spec.in: BuildRequires gcc-toolset-11-libatomic-devel for aarch64

to address following failure when generating the building system
using CMake:

```
-- Performing Test HAVE_LIBATOMIC
-- Performing Test HAVE_LIBATOMIC - Failed
CMake Error at cmake/modules/CheckCxxAtomic.cmake:66 (message):
  Host compiler /opt/rh/gcc-toolset-11/root/usr/bin/c++ requires libatomic,
but it is not found
```

Signed-off-by: Kefu Chai <tchaikov@gmail.com>