Enable outline-atomics by default on AArch64 FreeBSD

[tgross35]

Many aarch64 targets without LSE in the baseline enable the
outline-atomics feature, which uses runtime detection of LSE for its
faster atomic ops. This provides nontrivial performance improvements on
most hardware from the past decade, at a small cost to anything pre-LSE.

Clang does not currently have this enabled on FreeBSD (search for
IsAArch64OutlineAtomicsDefault there) but there doesn't seem to be any
specific reason not. Thus, enable it here.

Based on #144938 which needs to land first (this PR is only meant to cover the final commit).

try-job: dist-x86_64-freebsd

[tgross35]

This will make FreeBSD act the same as most other targets after #144938, which also has some more details (as does the Zulip discussion).

Cc target maintainers @asomers @MikaelUrankar, I split this off in case you would like to verify that this works since Clang doesn't have this enabled. Basically that means running some atomic ops with a debugger and making sure it uses the LSE fast path (details). Might be worth pinging some FreeBSD arm maintainers even if they don't work on Rust, in case they want to verify this or consider doing the same in Clang?

@bors2 try

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☀️ Try build successful (CI)
Build commit: d958a4b (d958a4b01d15d8f9e1cdf450fea6a6bcc7d0aced, parent: 556beb9ec72360512d0294eb0855c92fb2c20c88)