Make a nicer output when QBasicAtomicInteger is used with a wrong T

Before, we'd get just an error message that the size of the array was
negative. Now, for C++11 compilers, we get a better error message:

qbasicatomic.h:117:5: error: static assertion failed: Template parameter is not a supported integer on this platform
qbasicatomic.h:119:24: error: invalid use of incomplete type ‘struct QAtomicOps<long long unsigned int>’

Change-Id: I6b0792254c0dc6103a4a7608f2942d59cda07c00
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>