Bring back the fix for GCC atomic builtins

Bring back the fix for GCC atomic builtins

Commit d53cbfe6035f ("Update to LLVM 14.0.6") removed the patch that
adapts atomic.c to use GCC builtins instead of clang, however its
functionality is still required.
This change brings back this functionality in a cleaner and more
future-proof way, by providing a header that implements clang builtins
in terms of GCC ones.

Signed-off-by: Andrei Tatar <andrei@unikraft.io>
Reviewed-by: Stefan Jumarea <stefanjumarea02@gmail.com>
Reviewed-by: Maria Sfiraiala <maria.sfiraiala@gmail.com>
Reviewed-by: Razvan Deaconescu <razvand@unikraft.io>
Approved-by: Eduard Vintilă <eduard.vintila47@gmail.com>
Tested-by: Unikraft CI <monkey@unikraft.io>
GitHub-Closes: #12