x86: use gcc6'es flags asm() output support

x86: use gcc6'es flags asm() output support

..., rendering affected code more efficient and smaller.

Note that in atomic.h this at once does away with the redundant output
and input specifications of the memory location touched.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Kevin Tian <kevin.tian@intel.com>