Enable `outline-atomics` by default on aarch64 platforms without LSE

[tgross35]

Since #144938 and #149633, most Aarch64 platforms that don't have LSE in the baseline enable the +outline-atomics target feature. We keep getting more targets that fall into this boat, however, so it's probably worth making this the default.

I think we can probably just unconditionally enable outline-atomics on all aarch64 targets without checking LSE, since LLVM should ignore it if +lse is also set/implied.

Note that Apple targets would need an explicit disable until #149634 is fixed.

[JayanAXHF]

Is this still unimplemented?

[tgross35]

Yeah, it's still available. Note that we can't land it immediately because there is currently a problem with our outline atomics in compiler-builtins, but that should be resolved in the near future.

[yash-rana0101]

@rustbot claim