This way, the official atomic functions will be used when available,
rather than VLC's own implementation.

This commit also enables proper atomic operations on some platforms
or with some toolchains. __GCC_HAVE_SYNC_COMPARE_AND_SWAP_4 is not
defined on some platforms such as ARM Linux, nor by other compilers
than GCC such as LLVM/clang. Then atomic operations wrongly fell
back to the lame mutex-based implementation.

This will break support for some old compilers and old or irrelevant
instruction set architectures. They would need to provide replacement
for the Intel-originated __sync_* builtin functions.