Mark x86's memory barrier inline assembly as clobbering the cpu flags.

 Mark x86's memory barrier inline assembly as clobbering the cpu flags.
 
 x86's memory barrier assembly was marked as clobbering "memory" but
 not "cc" even though 'addl' sets various flags. As it turns out gcc on
 x86 implicitly assumes "cc" on every inline assembler statement, so
 it's not a bug. But as that's poorly documented and might get copied
 to architectures or compilers where that's not the case, it seems
 better to be precise.
 
 Discussion: 20140919100016.GH4277@alap3.anarazel.de
 
 To keep the code common, backpatch to 9.2 where explicit memory
 barriers were introduced.