Revert "lavu/atomic: add support for the new memory model aware gcc built-ins"

This reverts commit faa9d298.

This change became superfluous when support for C11 atomics was introduced.
Reverting it will make the removal of this implementation in an upcoming
merge conflict free.

Reviewed-by: wm4 <nfxjfg@googlemail.com>
Signed-off-by: James Almer <jamrial@gmail.com>