atomic asm snippets: Replace "memory" clobber with "Q" constraint

atomic asm snippets: Replace "memory" clobber with "Q" constraint

In our atomic asm snippets, using the "Q" constraint,
(https://gcc.gnu.org/onlinedocs/gcc/Machine-Constraints.html tells gcc
which exact memory is modified i.e. the bytes covered by the atomic variable),
is better than using the "memory" clobber (tells gcc that all memory is
modified, effectively a compiler memory barrier which prevents compiler
optimizations).