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.

This matches what Clang does 1.

Context: #144938

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

[rustbot]

These commits modify compiler targets.
(See the Target Tier Policy.)

[rustbot]

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[rust-bors]

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

[brad0]

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.

This comment is not true. I commited this 3 months ago.

[Driver] Enable outline atomics for FreeBSD/aarch64

[rustbot]

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

[tgross35]

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.

This comment is not true. I commited this 3 months ago.

[Driver] Enable outline atomics for FreeBSD/aarch64

Thanks for the link. This was split from #144938, the commit message predates the LLVM commit.

Any objections here then?

[tgross35]

r? @davidtwco as you handled the others