Use a .flag() with an if statement for mno-outline-atomics

When using .flag(), x86 build fails since the compiler doesn't know about no-outline-atomics.

When using .flag_if_supported(), aarch64 fails since the flag doesn't get applied because ????

Move to just .flag() with an if statement guard to aarch64

(this finally works for both aarch64 and x86_64)