atomic: implement atomic operations based on gcc's __sync extension

atomic: implement atomic operations based on gcc's __sync extension

Newer gccs and intel ccs support a __sync extension for making use of
atomic operations. This patch replaces the handcrafted x86 atomic
operation support with usage of __sync.

__sync is supported by more processors and by more compilers than the
old assembler code. Also, this extension has been available on gcc for
quite a while now for x86, so replacing the old assembler code should
only be a loss when very old compiilers are used.