x86/atomic: Improvements and simplifications to assembly constraints

x86/atomic: Improvements and simplifications to assembly constraints

 * Constraints in the form "=r" (x) : "0" (x) can be folded to just "+r" (x)
 * Switch to using named parameters (mostly for legibility) which in
   particular helps with...
 * __xchg(), __cmpxchg() and __cmpxchg_user() modify their memory operand, so
   must list it as an output operand.  This only works because they each have
   a memory clobber to give the construct full compiler-barrier properties.
 * Every memory operand has an explicit known size.  Letting the compiler see
   the real size rather than obscuring it with __xg() allows for the removal
   of the instruction size suffixes without introducing ambiguity.
 * Drop semicolons after lock prefixes.
 * Other misc style changes.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>